mount: make NAME=value tags usable for non-root

mount: make NAME=value tags usable for non-root

The libmount does not care if we set source or target, it's able to
swap it, but the mount.c function sanitize_paths() does not work as
expected if we set NAME=value as target. It means that

 $ mount LABEL=foo

does not work for non-root users (since 51e3530cdcb1d4f3ab91ae953ebc5adcdc5f9239, v2.24).

This patch also checks if source or target is specified more
than once.

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