Remove deadlock in rc caused by pwait waiting for itself.

The following situation can trigger the deadlock:
1) Long time ago a_service was started through rc.d
2) We want to restart a_service and issue service a_service restart
3) rc.subr reads current process PID (via file or process),
   sends TERM signal and runs pwait with PID harvested
4) a_service process dies very quickly so it's PID becomes available.
   It is possible that while original process was running,
   PID counter overflowed and pwait got assigned a_service's PID.

This patch ignores pid(s) to wait that are equal to pwait PID.

Reported by:	Dan McGregor, Boris Lytochkin
Submitted by:	Boris Lytochkin <lytboris at gmail.com>
Reviewed By:	0mp
MFC after:	2 weeks
PR:		218598
Differential Revision: https://reviews.freebsd.org/D28240

bin/pwait/pwait.1

@@ -32,7 +32,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 5, 2020
+.Dd January 21, 2021
 .Dt PWAIT 1
 .Os
 .Sh NAME
@@ -145,6 +145,10 @@ is not a substitute for the
 .Xr wait 1
 builtin
 as it will not clean up any zombies or state in the parent process.
+.Pp
+To avoid deadlock,
+.Nm
+will ignore its own pid, if it is provided as a process id to wait for.
 .Sh HISTORY
 A
 .Nm

bin/pwait/pwait.c

@@ -146,6 +146,10 @@ main(int argc, char *argv[])
 			warnx("%s: bad process id", s);
 			continue;
 		}
+		if (pid == getpid()) {
+			warnx("%s: skiping my own pid", s);
+			continue;
+		}
 		for (i = 0; i < nleft; i++) {
 			if (e[i].ident == (uintptr_t)pid) {
 				break;