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

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

tests now include if:
- start succeeds when the node name is given in a different args file than vm.args
- start fails when no node name given
- start fails when multiple node names given
- start fails when referenced args_file does not exist
- start fails when a referenced args_file is not readable
- start fails when an args_file is referenced via a relative path
- start fails when there are circular dependencies between args_files

Let people define their own when they want to.

Provide a status command to start script which,
by default, runs a builtin hook that simply prints
which applications are running in the node.
This hook can then be customized to print whatever
the user wants by adding
  {status, [{custom, "path/to/hook"}]}
to already existing extended_start_script_hooks.

Only ensure kernel,stdlib for empty dep apps

Two types are supported, direct chmoding, like

{chmod, 8#00700, "path/to/file/maybe/with/{{templates}}" }

or templating the permission where you have a template var like

{file_perm, 8#00700}

and an overlay

{chmod, "{{file_perm}}","path/to/file/maybe/with/{{templates}}" }

Do not try and ensure them for every app, there
are some OTP apps that only require kernel and
there's really no point in rewriting those.