util: rework strappenda(), and rename it strjoina()

After all it is now much more like strjoin() than strappend(). At the
same time, add support for NULL sentinels, even if they are normally not
necessary.

src/core/socket.c

@@ -266,7 +266,7 @@ static int socket_add_device_link(Socket *s) {
         if (!s->bind_to_device || streq(s->bind_to_device, "lo"))
                 return 0;
 
-        t = strappenda("/sys/subsystem/net/devices/", s->bind_to_device);
+        t = strjoina("/sys/subsystem/net/devices/", s->bind_to_device);
         return unit_add_node_link(UNIT(s), t, false);
 }
 
@@ -473,7 +473,7 @@ static void socket_dump(Unit *u, FILE *f, const char *prefix) {
         assert(f);
 
         prefix = strempty(prefix);
-        prefix2 = strappenda(prefix, "\t");
+        prefix2 = strjoina(prefix, "\t");
 
         fprintf(f,
                 "%sSocket State: %s\n"