* lib-src/emacsclient.c: Disable -Wformat-truncation=2,
to avoid false alarms about the new snprintf calls.
(local_sockname): New function.
(set_local_socket): Use it. Prefer XDG_RUNTIME_DIR (if set)
for location of socket directory. Avoid unnecessary memory
allocation by using snprintf to destination.
* lisp/server.el (server-socket-dir): Prefer XDG_RUNTIME_DIR if set.