Explicitly initialize sigaction::sa_mask members with sigemptyset(3)

Not doing so before calling sigaction(3) is "undefined" according to
POSIX[1]:

> Applications shall call either sigemptyset() or sigfillset() at least
> once for each object of type sigset_t prior to any other use of that
> object. If such an object is not initialized in this way, but is
> nonetheless supplied as an argument to any of pthread_sigmask(),
> sigaction(), sigaddset(), sigdelset(), sigismember(), sigpending(),
> sigprocmask(), sigsuspend(), sigtimedwait(), sigwait(), or
> sigwaitinfo(), the results are undefined.

The use of designated initializers means that sa_mask members were
still being initialized, but sigset_t is an opaque type and implicit
initialization doesn't necessarily produce the same results as using
sigemptyset(3) (although it typically does on most implementations).

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaddset.html

client.c

@@ -455,7 +455,8 @@ main(int argc, char *const *argv)
         }
     }
 
-    const struct sigaction sa = {.sa_handler = &sig_handler};
+    struct sigaction sa = {.sa_handler = &sig_handler};
+    sigemptyset(&sa.sa_mask);
     if (sigaction(SIGINT, &sa, NULL) < 0 || sigaction(SIGTERM, &sa, NULL) < 0) {
         LOG_ERRNO("failed to register signal handlers");
         goto err;