umount: instead of non-standard -D, use -d with opposite meaning

  (closes bug 1604)
umount: do not try to free loop device or erase mtab if remounted ro
umount: do not complain several times about the same mountpoint

function                                             old     new   delta
umount_main                                          646     638      -8
packed_usage                                       23662   23652     -10
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-18)             Total: -18 bytes

1 files changed, 1 insertions, 1 deletions

diff --git a/include/usage.h b/include/usage.h
index df15dbb46..061cb5568 100644
--- a/include/usage.h
+++ b/include/usage.h
@@ -3996,7 +3996,7 @@ USE_FEATURE_RUN_PARTS_FANCY("\n	-l	Prints names of all matching files even when
        "\n      -l      Lazy umount (detach filesystem)" \
        "\n      -f      Force umount (i.e., unreachable NFS server)" \
         USE_FEATURE_MOUNT_LOOP( \
-       "\n      -D      Do not free loop device (if a loop device has been used)")
+       "\n      -d      Free loop device if it has been used")
 #define umount_example_usage \
        "$ umount /dev/hdc1\n"