Fix a race in echo-signal

Sadly a kind of mistake of which I'm well aware these days;
previously, you can imagine that we pause, some other signal gets
delivered, then, after we test sig != got but before we re-invoke
pause, our desired signal arrives.  Then, we hang in pause until
another signal arrives (an arbitrarily long time).

Also, might as well write our message without using buffered IO, to be
safe.

helpers/echo-signal.c

@@ -12,8 +12,6 @@ exec ${CC:-cc} ${CFLAGS:--Wall -Wextra -g} $0 -o $1
 #include <string.h>
 #include <unistd.h>
 
-static sig_atomic_t got;
-
 static int int_of_signal_name(char *name)
 {
     if (!strcmp(name, "INT")) return SIGINT;
@@ -23,16 +21,18 @@ static int int_of_signal_name(char *name)
     abort();
 }
 
-static void handler(int n) { got = n; }
-
 int main(int argc, char **argv)
 {
-    struct sigaction action = { .sa_handler = handler };
     if (2 != argc) abort();
     int sig = int_of_signal_name(argv[1]);
-    sigaction(sig, &action, NULL);
-    puts("ready");
-    do { pause(); } while (sig != got);
+
+    sigset_t set, prev;
+    sigemptyset(&set);
+    sigaddset(&set, sig);
+    sigprocmask(SIG_BLOCK, &set, &prev);
+    write(1, "ready\n", 6);
+    int got;
+    do { sigwait(&set, &got); } while (sig != got);
     puts(argv[1]);
     return 0;
 }