Run the privileged dhclient process in its own session.

In the MPSAFE TTY branch, I noticed PTY's to be leaked, because
dhclient's privileged process was run inside the session of, say, the
login shell. Make sure we call setsid() here.

Approved by:	philip (mentor), brooks

sbin/dhclient/dhclient.c

@@ -2612,6 +2612,7 @@ fork_privchld(int fd, int fd2)
 
 	setproctitle("%s [priv]", ifi->name);
 
+	setsid();
 	dup2(nullfd, STDIN_FILENO);
 	dup2(nullfd, STDOUT_FILENO);
 	dup2(nullfd, STDERR_FILENO);