libcasper: reap zombies without requiring waitpid(2)

Reported and tested by:	bapt
Reviewed by:	bapt, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59348

lib/libcasper/libcasper/libcasper.c

@@ -95,7 +95,7 @@ cap_init(void)
 		return (NULL);
 	}
 
-	pid = pdfork(&pfd, 0);
+	pid = pdfork(&pfd, PD_NOWAITPID);
 	if (pid == 0) {
 		/* Child. */
 		close(sock[0]);

lib/libcasper/libcasper/zygote.c

@@ -149,7 +149,7 @@ zygote_main(int *sockp)
 			error = errno;
 			goto send;
 		}
-		pid = pdfork(&procfd, 0);
+		pid = pdfork(&procfd, PD_NOWAITPID);
 		switch (pid) {
 		case -1:
 			/* Failure. */