repart: add --append-fstab parameter (#37238)

If --generate-fstab=PATH is used, there is the possibility that the
fstab file already exists, making systemd-repart fail.

This commit will add a new --append-fstab= parameter, that will read
the file and merge it with the new generated content.  Using the
comments, the command can separate the automatic-generated section from
the user-provided section, allowing for the next append the replacement
only of the automatic-generated section, keeping the user one.