path-util: add generic `ignore` extension to the hidden/backup files

path-util: add generic `ignore` extension to the hidden/backup files

All currently ignored extensions either “belong” to some other program (and may
thus be used, changed or interpreted by that) or imply a certain meaning (like
`bak`, `old` and `new`).

This adds the more generic extension `ignore` which is meant to imply no purpose
whatsoever.

A use case would be the following scenario:

One might want to share a drop-in configuration over multiple units like in:
`/etc/systemd/system/a.service/foo.conf` and
`/etc/systemd/system/b.service/foo.conf`, which then would be symbolic links to
the actual file.

Of course one could place that actual file in a directory that is not parsed by
systemd, but it is unit-configuration after all, so it would be nice to have it
somewhere beneath `/etc/systemd/system` (or other parsed directories).

However, placing it there needs a way to ensure that it’s never accidentally
used by systemd, which this extension is meant for.