Changes parse_git url function to use Using RFC3986 standard to validate
git uri instead of matching strings in function head. Also accepts scp
style syntax for parsing.

`template` is not a kv-style argument, but a plain argument. If you give `template=app` on the command line, you get "template not found."

In the spirit of Original Rebar's "require_min_otp_vsn", this adds
rebar_utils:check_min_otp_version/1 (taking a string containing the
minimum version) and rebar_utils:check_blacklisted_otp_versions/1
(taking a list of regular expression strings), as well as tests in
rebar_utils_SUITE. They're currently only called by the tests-
how/where to best place calls to them from non-test code needs to be
determined (at which point two corresponding rebar.config keys can be
supported). For example, the version probably shouldn't be enforced when
just running "rebar3 help".

Linking message will only be printed the first time the link or copy
actually happens.

Note:

rebar_file_utils:symlink_or_copy will now return 'exists'
instead of ok, if it did nothing because the link exists.

Nothing was checking the return value yet, so seemed reasonable.

Basic escaping is done only. Fancy hex sequences are not covered, but
this should otherwise take care of the most common issues.

Fixes #497

This reverts commit b06a6ecadd.