libmount: allow to disable swap between source and target

libmount: allow to disable swap between source and target

In some cases (for example if only one mount argument is given) may be
mount request ambivalent:

	# mount /foo

and fstab:

	/dev/sda5	/foo	rw	0	0
	/foo		/bar	bind	0	0

The libmount allows to swap between source and target (if
source is not LABEL or UUID) by default. The new function

	mnt_context_disable_swapmatch()

allows to disable this feature.

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