Updating the bootstrap script in #1689 made it so symlinks that already
exist return `exist` rather than ok, in line with the regular code.
However the bootstrap module wouldn't handle this case.

This fixes the problem by ensuring that `exists` is as valid as `ok`.

The bootstrap script has been mostly unchanged for a long period of
time, and has not benefited from all the changes and improvements that
rebar3 itself had, including (but not limited to) path escaping,
handling of directories on windows, and edge case management when it
comes to file and symlink handling.

This patch brings the updates seen in rebar_string_utils, rebar_utils,
and rebar_dir into the bootstrap script so that fewer people have build
issues when starting from source, from scratch.

Updates relx (windows fixes) and erlware commons (strings)

- OTP-21 readiness, Full Unicode support, massive dep upgrade
- fixed handling of proxy username and password when fetching registry
- git versions from tag made consistent and all strip 'v' prefix
- Prevent hard crash on duplicate plugin paths
- Fix include paths in profile multiapp edge case
- Fix unlock state carry, which broke do sequences with unlock in them.
- Avoid guessing on utf8 decoding of app files
- Various fixes related to .app files
- Warn user when an unsupported local git or hg resource is used
- Corrects a fix to src_dir values
- Update eunit_formatters to latest version
- Changes in wording of warnings for more accuracy

This replaces all deprecated function usage by alternative ones based on
a version switch enacted at compile time, preventing all warnings.

This will likely introduce some possible runtime errors in using a
Rebar3 compiled on OTP-20 or OTP-21 back in versions 19 and earlier, but
we can't really work around that.

A bunch of dependencies have been updated to support OTP-21 without
warnings as well.

Get more up-to-date root certs. This was blocked for a while because
certifi wouldn't build on windows, but this is now fixed, with minimal
changes to the use case -- only a small change in the bootstrap script
is required.

The new certifi lib is also a few megabytes lighter than before, which
is good for rebar3.

Includes Relx fixes

Signed-off-by: Fred Hebert <mononcqc@ferd.ca>

Patches:

- Try to start epmd when distribution fails
- Allow non-default profile deps upgrade
- Switch default license in templates to Apache 2.0, and hook it up in
  the .app file
- Fix git SHA comparisons for dep update
- Let relx know about rebar-specified erl_opts
- Use rm_rf instead of ec_file:delete, fixing some deps-related issues
  on Windows
- Handle internal CT failures
- Fix profile merging of erl_first_files options
- Fix CT output in skipped test cases and failing end of function
  hooks
- Officially drop R15 support, since its SSL libs can no longer talk
  to hex.pm
- REBAR_COLOR supports all ec_cmd_log intensity options, allowing the
  none value to disable all colors without dropping termcap support
- Bump relx to 3.22.4

The one new feature supported here is for the interactive shell:

- Support commands in the shell such as r3:Command(),
  r3:Command("--args=as a string"), or r3:Command(Namespace, "--args=as a
  string"), instead of just r3:do(Command).

- [OSX tests added to CI](https://github.com/erlang/rebar3/pull/1471),
joining linux and windows 10
- [upgrade relx to 3.22.3](https://github.com/erlang/rebar3/pull/1518),
[fixing multiple
issues](https://github.com/erlware/relx/releases/tag/v3.22.3)
- [Fixing eunit errors in 19.3 when `init_per_*` isn't paired with
`end_per_*`](https://github.com/erlang/rebar3/pull/1516)
- [Allow single test spec in
ct_opts](https://github.com/erlang/rebar3/pull/1506)
- [Properly warn when missing inets at
runtime](https://github.com/erlang/rebar3/pull/1505)
- [rebar_utils:escape_chars handles quotes in
paths](https://github.com/erlang/rebar3/pull/1504)
- [Fix typo in help text for upgrade
command](https://github.com/erlang/rebar3/pull/1503)
- [change package version warnings to debug
level](https://github.com/erlang/rebar3/pull/1502)
- [Path-type sensitive .cmd creation for
escripts](https://github.com/erlang/rebar3/pull/1500)
- [Windows escripts get cmd
autogenerated](https://github.com/erlang/rebar3/pull/1494)
- [REBAR_CONFIG impacts config file for project root
only](https://github.com/erlang/rebar3/pull/1497)
- [Fix wildcard usage in rebar3
clean](https://github.com/erlang/rebar3/pull/1498)
- [Bump cth_readable to
1.2.4](https://github.com/erlang/rebar3/pull/1496) (fixing some
erroneous reports when suites get skipped)
- [Add respect for src_dirs option in app_discover and
app_info](https://github.com/erlang/rebar3/pull/1486)
- [Add support for global plugin-defined
templates](https://github.com/erlang/rebar3/pull/1473)
- [catch badarg exception from eunit in
`rebar_prv_eunit`](https://github.com/erlang/rebar3/pull/1472)

This also patches up a problem for hooks when dealing with directories
with spaces in them, and reduces complexity of bootstrap file.

This detects existence of Hex registry at
$HOME/.cache/rebar3/hex/default/registry and skips "rebar3 update" step.
It also detect presence of bootstrap dependencies in _build/default/lib/
and skips fetching them.

fix #790