namespace-util: introduce userns_acquire() as helper for allocating new unbound userns

This returns a namespace fd, and takes a uidmap/gidmap as string. This
is split out out mount-util.c's remount_idmap() logic, so that we can
allocate a userns independently.

src/basic/namespace-util.h

@@ -24,3 +24,5 @@ static inline bool userns_shift_range_valid(uid_t shift, uid_t range) {
 
         return true;
 }
+
+int userns_acquire(const char *uid_map, const char *gid_map);