Make fsck ignore mounted filesystems if given the -M option

Make fsck ignore mounted filesystems if given the -M option

Adapted from the SuSE patch, but fixes a number of very serious
problems with the patch in SLES:

1) This changeset uses -M instead of -m; most lowercase options are
reserved for use by the filesystem-specific fsck programs.  All new
fsck options must be upper case.

2)  This changeset will skip the root filesystem in "fsck -AM", which
the SLES patch will not do.

3)  Loading /proc/mounts into the fs_info can cause -t opts matching to
malfuction.  So this changeset uses a simplified version of the
ismounted.c function from the ext2fs library.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>