Convert script settings to lists

Convert script settings to lists

With `mkosi.conf.d`, `mkosi.presets` and/or `[Match]`, it's become
really easy to split the configuration of an image in logical parts. It
might be done to isolate some settings that go together or to share the
same configuration across several images.

However, for each `mkosi.prepare`, `mkosi.build`, `mkosi.postinst` and
`mkosi.finalize` step, only one script could be executed as the one with
highest priority overrides the others.

Each script is now managed as a list of executables so that it's also
possible to split them if desired.

Backward compatibility is kind of maintained by keeping the same options
on the command line and by introducing aliases for the renamed INI
settings (with a deprecation warning). These aliases also parse the
settings as a list, so compatibility is still broken when a script was
expected to override a previously defined one, as they are now both
executed.