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

slave.c

@@ -275,7 +275,8 @@ slave_spawn(int ptmx, int argc, const char *cwd, char *const *argv,
         }
 
         /* Restore signal mask, and SIG_IGN'd signals */
-        const struct sigaction dfl = {.sa_handler = SIG_DFL};
+        struct sigaction dfl = {.sa_handler = SIG_DFL};
+        sigemptyset(&dfl.sa_mask);
         sigset_t mask;
         sigemptyset(&mask);