namespace-util: introduce netns_acquire()

Similar to userns_acquire(), but for network namespace.

src/basic/namespace-util.h

@@ -51,4 +51,5 @@ 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 netns_acquire(void);
 int in_same_namespace(pid_t pid1, pid_t pid2, NamespaceType type);