update umount procedure

update umount procedure

If umount(8) supports --fake and --no-canonicalize (util-linux-ng
version 2.18 or later), and umount(2) supports the UMOUNT_NOFOLLOW
flag (linux kernel version 2.6.35 or later) then, "fusermount -u" will
call the umount(2) system call and use "umount --fake ..." to update
/etc/mtab

Added --disable-legacy-umount option to configure.  This disables the
runtime checking of umount(8) version.  When built with this option
then "fusermount -u" will fail if umount(8) doesn't support the --fake
and --no-canonicalize options.