ismounted.c (check_mntent, check_mntent_file): Add better

ismounted.c (check_mntent, check_mntent_file): Add better
	support for loopback-mounted filesystems.  Check /etc/mtab
	if /proc/mounts doesn't turn up any mount flags, since
	/etc/mtab has the loopback image filename, instead of
	/dev/loop0.  Also, check based on st_dev and st_ino, so
	that if a relative pathname or a pathnames using symbolic
	links are used, we can detect the the filesystem correctly
	in those cases.  (Addresses Sourceforge bug #619119)