mount: Move fstypes related code into separate files

After this the mount.c is no longer the fattest .c in the tree:

  - before
    3947 criu/mount.c
    3179 criu/cr-restore.c
    2622 criu/proc_parse.c
    2095 criu/tty.c

  - after
    3179 criu/cr-restore.c
    3170 criu/mount.c
    2623 criu/proc_parse.c
    2095 criu/tty.c

One "not move" thing is -- since fstype array is static to the
filesystems.c and mount.c needs to access fstype[1] for auto
mounts, the fstype_auto() call is added.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>