fstab-generator: fix options in systemd.mount-extra= arg

fstab-generator: fix options in systemd.mount-extra= arg

Fixes a bug introduced by 55365b0a233ae3024411fd0815ad930e20f6a3d6 (v254).

The arguments `(rd.)systemd.mount-extra` take a value that looks like
`WHAT:WHERE[:FSTYPE[:OPTIONS]]`. The `OPTIONS` were parsed into a nulstr
where a comma-separated c-string was expected. This leads to a bug where
only the first option was taken into account by the generator.

For example, if you passed `systemd.mount-extra=/x:/y:baz:ro,defaults`
to the kernel, `systemd-fstab-generator` would translate that into a
nulstr: `ro\0defaults\0`.
Since methods processing options in the generator expected a
comma-separated c-string, they would only see the first option, `ro` in
this case.