umount: fix "umount -t FSTYPE MNTPOINT" acting as if -a is specified

While at it, add -t to --help, and fix comments which say that -t is ignored

function                                             old     new   delta
packed_usage                                       32427   32444     +17
umount_main                                          558     552      -6
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(add/remove: 0/0 grow/shrink: 1/1 up/down: 17/-6)              Total: 11 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed