setup_mount_entries: ignore mount failure if 'optional'

If 'optional' is in the mount options, then avoid failure in
mount().

Experiments suggest we could just do this checking data at
mount_entry(), but that feels less proper than using
hasmntopt() against the mntent.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>