util-lib: add new ifname_valid() call that validates interface names

Make use of this in nspawn at a couple of places. A later commit should port
more code over to this, including networkd.

src/basic/socket-util.h

@@ -123,6 +123,8 @@ int fd_inc_rcvbuf(int fd, size_t n);
 int ip_tos_to_string_alloc(int i, char **s);
 int ip_tos_from_string(const char *s);
 
+bool ifname_valid(const char *p);
+
 int getpeercred(int fd, struct ucred *ucred);
 int getpeersec(int fd, char **ret);