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/basic/unit-def.c

@@ -15,7 +15,7 @@ char *unit_dbus_path_from_name(const char *name) {
         if (!e)
                 return NULL;
 
-        return strappend("/org/freedesktop/systemd1/unit/", e);
+        return strjoin("/org/freedesktop/systemd1/unit/", e);
 }
 
 int unit_name_from_dbus_path(const char *path, char **name) {