Introduced while merging PRs 445, 541

Provide a new config directive that allows per-app module exclusion

By introducing a new entry in the config file allow excluding
specific modules from a given app, they will not be copied
onto the final release and their reference removed from
the .app file. The new entry takes on the following form:
{exclude_modules, [
  {App :: atom(), [Module :: atom()]}
]}

New 'extended_start_script_hooks' directive that allows the
developer to define six different hook shell scripts to be invoked
at pre/post start/stop/install upgrade phases.
Besides these custom defined scripts, other types of builtin
scripts are also available, these offer pre-packaged functionality
that can be used directly, they are:
  pid - writes the beam pid to a configurable file location
          (/var/run/<rel_name>.pid by default).
  wait_for_vm_start - waits for the vm to start (ie. when it responds to pings)
  wait_for_process - waits for a configurable name
                                to appear in the erlang process registry
The hook scripts are invoked with the 'source' command, therefore
they have access to all the variables in the start script.

The first run would correctly replace the environment
variables, however it would also overwrite the original
vm.args and sys.config thus preventing any further
substitution in subsequent runs.
Dev mode runs were also broken, all runs after the
first were required to also define the
RELX_REPLACE_OS_VARS variable in order not to
overwrite the current vm.args with the original one,
this prevented simply attaching to an already running
node that was started this way.

Add tests to exercise this functionality.

This reverts commit 701da7b9f2dfb872351b56938f7aed5bff0d79c7.

Remove redundant check for extended_start_script.
Add tests that enforce this invariant.

Using the include_src option