fix errors in sigqueue (potential information leak, wrong behavior)

1. any padding in the siginfo struct was not necessarily zero-filled,
so it might have contained private data off the caller's stack.

2. the uid and pid must be filled in from userspace. the previous
rsyscall fix broke rsyscalls because the values were always incorrect.

src/signal/sigqueue.c

@@ -5,10 +5,12 @@
 
 int sigqueue(pid_t pid, int sig, const union sigval value)
 {
-	siginfo_t si = {
-		.si_signo = sig,
-		.si_code = -1,
-		.si_value = value,
-	};
+	siginfo_t si;
+	memset(&si, 0, sizeof si);
+	si.si_signo = sig;
+	si.si_code = SI_QUEUE;
+	si.si_value = value;
+	si.si_pid = getpid();
+	si.si_uid = getuid();
 	return syscall3(__NR_rt_sigqueueinfo, pid, sig, (long)&si);
 }