discover/device-handler: Attempt to retry failed mounts

discover/device-handler: Attempt to retry failed mounts

Commit 6c1a9dd, "discover: Allow fs recovery if snapshot available",
forced the use of 'norecovery' for all XFS mounts to avoid failing when
a cross-endian journal existed. This is a bit heavy handed, healthy XFS
file systems can still be safely mounted, as can dirty filesystems in
the same endian as Petitboot.

This adds try_mount() which opportunistically mounts devices and falls
back to using 'norecovery' where possible on failure. This enables XFS
filesystems to be mounted read-write when possible. try_mount() contains
the logic previously described by fs_parameters(), and should be used in
place of any existing calls to mount().

Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>