LU-10966 utils: Fix `lfs check` documentation and arguments

LU-10966 utils: Fix `lfs check` documentation and arguments

Several bugs exist for 'lfs check'. Add checking for MGS servers.
Add 'lfs check mdts' which is inline with the other options but
still keep mds for backwards compatibility. Do the same with the
'all' option which is equivalent to older 'servers' that is also
kept for backwards compatibility. Update man pages to reflect
these changes

Change-Id: Ifffec8fd1498616036122f9af311cd6366d693bc
Signed-off-by: George Melikov <mail@gmelikov.ru>
Reviewed-on: https://review.whamcloud.com/33775
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>