Add -o loop= option to mount, and fix error detection in mount and umount

This fixes bug http://b.android.com/18419, which complains about a bogus
error check in the mount command (it also was wrong in the umount command)
and also asks for the mount command to support more than one loopback
device, as mentioned in the FIXME comments in mount.c.  This required some
corresponding changes to umount.c

Change-Id: Ib796c70926395e61557e487bad64984d3295d5f3