Audit slstatus.c

Audit slstatus.c

 1) Remove setlocale() (locales are harmful and any 'issues' shall
    be fixed in different ways that are expected).
 2) Disable buffering on stdout with setbuf() rather than flushing
    it each time.
 3) Make error messages more consistent.
 4) Add error checks where applicable.
 5) Make code a bit more readable where res is assigned.
 6) Use XFlush() rather than XSync() (we don't need to wait for the
    XServer to react, which could lead to long hangs on our side).