HBSD: pull in upstream fix for pwait hang when watching its own pid

From the FreeBSD Bug 218598:

  pwait will deadlock when passed its own PID.
  I experienced this when kern.randompid was set with poudriere.
  The code was more or less this:

  #!/bin/sh
  do_the_thing&
  pwait $!
  do_the_other_thing

  If do_the_thing ends before pwait starts, and do_the_thing and
  pwait get the same pid, pwait waits indefinitely.

FreeBSD-PR: 218598
FreeBSD-PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218598
MFC-to: 10-STABLE
MFC-to: 11-STABLE
X-MFC-with: baf6bf5b16681f98941f25302074ff64df89e076
Submitted-by: Dan McGregor <[email protected]>
Signed-off-by: Oliver Pinter <[email protected]>
(cherry picked from commit e0b45ab3bc9443689c2d95ce2e3b552a027f5494)
Signed-off-by: Oliver Pinter <[email protected]>
(cherry picked from commit 81c3c9c)
Signed-off-by: Oliver Pinter <[email protected]>

bin/pwait/pwait.c

@@ -71,6 +71,7 @@ main(int argc, char *argv[])
 	long pid;
 	char *s, *end;
 	double timeout;
+	pid_t me;
 
 	tflag = verbose = 0;
 	memset(&itv, 0, sizeof(itv));
@@ -118,6 +119,8 @@ main(int argc, char *argv[])
 	if (argc == 0)
 		usage();
 
+	me = getpid();
+
 	kq = kqueue();
 	if (kq == -1)
 		err(1, "kqueue");
@@ -136,6 +139,10 @@ main(int argc, char *argv[])
 			warnx("%s: bad process id", s);
 			continue;
 		}
+		if (pid == me) {
+			warnx("%s: ignoring own process id", s);
+			continue;
+		}
 		duplicate = 0;
 		for (i = 0; i < nleft; i++)
 			if (e[i].ident == (uintptr_t)pid)