mount: handle filesystems with subtype

mount: handle filesystems with subtype

Linux can handle filesystem types with "MAINTYPE.SUBTYPE" format,
where the main type determines the actual filesystem driver while the
subtype can be interpreted by the filesystem itself.

When searching for mount helpers mount(8) and umount(8) should also
interpret such types, falling back to (u)mount.MAINTYPE if
(u)mount.MAINTYPE.SUBTYPE doesn't exist.

This patch implements this, passing the type with "-t TYPE"
to the mount program in this case.

Reported-by: Josef Bacik <josef@redhat.com>
Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=625064
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>