When no release specified in cli, prevent overwriting default_release
in relx config.

* support for OTP21's sys.config.src file in releases

* always replace os vars in .src files if found

* support vm_args_src to be consistent with sys_config_src

* add newlines after warning logs

* improve sys and vm src config tests

Had a problem in OTP-21 (master) where escript calls in nodetool would
fail since the file is not around and is being used by default there.

The patch works by copying its equivalent file, which I belive to be
just the start_clean file (their .rel.src are identical), into the same
directories as we do for start_clean.

Internal errors are renamed to be made neutral.

Use either optional compilation or version-safe variants of the string
functions. Prevents warnings when the switch to OTP-21 will happen.

Pass prefix through

Useful for copying custom files from an erts build
such as a .kerl.config file.

* Extended start script command extensions

Provide a mechanism that allows for the application
to extend the list of commands available to be invoked
from the start script. An application may be able to
define a 'foo' extension that is associated with a 'foo_script'
written and maintained by the applicationr, (this association is
kept in rebar.config), upon invocation of bin/<release_name> foo
the 'foo_script' will then be invoked.

* Add test coverage for extension script

* Ensure extended script usage argument