namespace-util: add in_same_namespace()

Add a helper for the canonical way to determine whether two namespaces
are identical.

src/basic/namespace-util.h

@@ -45,3 +45,4 @@ static inline bool userns_shift_range_valid(uid_t shift, uid_t range) {
 }
 
 int userns_acquire(const char *uid_map, const char *gid_map);
+int in_same_namespace(pid_t pid1, pid_t pid2, NamespaceType type);