mount: automatically detect and loop-mount regular files

mount: automatically detect and loop-mount regular files

This patch allows to automatically create a loop device from a regular
file if a filesystem type is not specified, for example:

   mount /path/disk.img /mnt

If the filesystem type is specified than "-o loop" is required.

Note that there is not a restriction (on kernel side) that prevents
regular file as a mount(2) source argument. A filesystem that is able
to mount regular files could be implemented.

Based on a patch from Adam Jackson <ajax@redhat.com>.

Signed-off-by: Karel Zak <kzak@redhat.com>