stat-util: teach statx_mount_same() STATX_MNT_ID_UNIQUE

So far statx_mount_same() assumed STATX_MNT_ID_UNIQUE didn't exist.
However it does exist, hence do something useful if we see it set. Note
that this creates a certain ambiguity: if we compare one statx struct
with STATX_MNT_ID_UNIQUE and one without it (but with the regular mnt
id), then we cnanot really come to a clear conclusion, hence need to
introduce a third, unknown state.

Note that we don't request STATX_MNT_ID_UNIQUE yet wherever we call
statx_mount_same(). THis will be added in a later commit.

src/basic/stat-util.h

@@ -110,7 +110,7 @@ bool stat_inode_same(const struct stat *a, const struct stat *b);
 bool stat_inode_unmodified(const struct stat *a, const struct stat *b);
 
 bool statx_inode_same(const struct statx *a, const struct statx *b);
-bool statx_mount_same(const struct statx *a, const struct statx *b);
+int statx_mount_same(const struct statx *a, const struct statx *b);
 
 int xstatfsat(int dir_fd, const char *path, struct statfs *ret);