Add cleanup script via --cleanup file argument

[asoc]

Use case example:
During execution a temporary Docker image is loaded and started, but
must be stopped and removed before returning to the controlling
terminal. Since this must happen in both exit cases (normal exit, and
signal capture handling) and since trap handling does not appear to work inside
startup_script, an alternative is needed.

Specifying makeself ... --cleanup ./cleanup.sh ... allows using an
included script to stop and remove the image in both situations.

The cleanup script is provided the same arguments that are given to
startup_script and is executed from the same working directory.

[megastep]

Looks good, thanks!

[asoc]

Hey @megastep I did a bit more work using this change and discovered that the $@ arguments aren't actually passed correctly to the cleanup script. After fiddling for a couple hours I can't figure out how to get them passed while also preserving spaces in arguments, so I think readjusting the code (and --help line) to just

        eval "\"\$cleanup_script\" \$scriptargs"

would be the best path. Would you prefer another PR or think you might want to take a stab at it?

[megastep]

You can make another PR if you'd like. Do you have a way to distinguish between arguments to the startup and cleanup scripts?

[asoc]

Could probably add something like --cleanup-args arg and pass the arg value along. Tested a quick change and this seems to work:

./test.run --cleanup-args '--arg1 someval --arg2 "val with space"'  -- --script-arg1 ...

If that works for you I'll make a new PR with both changes.