tree-wide: get rid of strappend()

It's a special case of strjoin(), so no need to keep both. In particular
as typing strjoin() is even shoert than strappend().

src/nspawn/nspawn-settings.c

@@ -505,7 +505,7 @@ int config_parse_network_zone(
         assert(lvalue);
         assert(rvalue);
 
-        j = strappend("vz-", rvalue);
+        j = strjoin("vz-", rvalue);
         if (!ifname_valid(j)) {
                 log_syntax(unit, LOG_ERR, filename, line, 0, "Invalid network zone name, ignoring: %s", rvalue);
                 return 0;