libmount: make libmount.h consistent with code

The function with nonnull attribute are silently optimized by gcc so
all "if (foo)" are removed if the "foo" is expected as non-null. It
make the code less usable and robust in some situations.

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: add mnt_table_find_mountpoint()

This is more robust implementation of mnt_get_mountpoint() that does
not ignore bind mountpoints (mount --bind /mnt /mnt) as it does not
depend on st_dev numbers.

Signed-off-by: Karel Zak <kzak@redhat.com>
mount: improve --move error message on shared trees

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