Error report killing inet_gethost_native after eunit

Avoid error reports like this one:

  prompt$ rebar eunit
  ==> dummy (eunit)
    Test passed.

  =ERROR REPORT==== 29-Dec-2011::23:22:11 ===
  ** Generic server inet_gethost_native_sup terminating
  ** Last message in was {'EXIT',<0.62.0>,killed}
  ** When Server state == {state,inet_gethost_native,undefined,<0.62.0>,
                                 {local,inet_gethost_native_sup}}
  ** Reason for termination ==
  ** killed

Previously, it could happen if an eunit test did something that
required a name lookup, like in this example:

  -module(dummy_tests).
  -include_lib("eunit/include/eunit.hrl").

  x_test() ->
      {ok, _Hostent} = inet:gethostbyname(localhost).

The inet_gethost_native is a process, started on demand, under a
supervisor_bridge under the kernel_sup, but it is not a gen_server
process or the like, so it has nothing in '$ancestors' in its process
dictionary to indicate it is part of kernel.

This patch allows users to specify the directory in which common_test
source files can be found. Most common_test suites are integration,
rather than unit tests and keeping the sources apart from test sources
for other frameworks such as eunit and PropEr is a useful feature.

It is now possible to call rebar as:

  rebar xref skip_app=Mod1,Mod2,...

This makes it easy to skip running xref on (e.g)
imported dependencies in your application.

The function rebar_utils:is_skipped_app/0 is added so
that other rebar commands may use it.

Previously if erl_interface could not be found {error, not_found} was
incorrectly used as a path. With this change we fail early with a
descriptive error message.

{pre_hooks,  [{"linux", compile, "c_src/build_linux.sh"}]}.
{post_hooks, [{"linux", compile, "c_src/build_linux.sh clean"}]}.

This patch adds support for first_files to the port_compiler, via the
new `port_first_files` rebar config element.

When generating a node using rebar generate, rebar always used
code:root_dir() to find the Erlang installation to clone into the node
tree.  However, for anyone wishing to build a cross-compiled node, there
was no way to override this.  This patch adds a new option to
reltool.config file to allow an alternate root_dir to be specified, for
example:

{root_dir, "/tmp/otp_R14B03_armv7l"}

This assumes that the contents of /tmp/otp_R14B03_armv7l has been
generated using the instructions found in the xcomp directory in your
$ERL_TOP structure.  NOTE: you may have to add additional filters to
exclude files not present in the xcomp version, such as the .smp files
if you have disabled that.

This means that applications get a chance to shut down themselves,
before rebar will start killing their processes.  This avoids ERROR
REPORTS from application supervisors.

The function may fail with a badarg exception because the first regex
returns an iolist() which is allowed to be a improper list. In this case
'++' cannot append to the iolist. The correct way to append something to
an iolist() is

  [iolist(), "tail"]

because iolist's are allowed to be arbitrarily deep lists.

The introduction of setup_env as a global concept caused the rebar_port_compiler
implementation to start getting called a LOT. The expansion of environment variables
that happens in the port compiler was O(n^n), which means you could see upwards of
80k invocations of lists:foldl on a single app "./rebar clean". This commit reworks
the expansion to be O(n^2), and reduces the running time for the same operation by
60%+. On a large project like Riak, the end result is that a build went from 200
seconds to 73.

Simple feature, useful to make link to something without copying it.