fsck: Allow to specify the fsck repair option in the cmdline

Some unattended systems do not have a console attached and entering
the default rescue mode will not be too helpful. Allow to specify
the "-y" option to attempt to fix all filesystem errors.

Manually verified by downloading an image.gz of e2fsprogs, using
losetup and running systemd-fsck on the loop device and varying
the fsck.repair=preen|yes|no option.