Create dhclient pid directory if it doesn't exist

- Upgrading from older FreeBSD versions can result in errors

 - /var/run can be a tmpfs, and this should be handled correctly

Approved by:		markj
Differential Revision:	https://reviews.freebsd.org/D28843
MFC after:		2 weeks

libexec/rc/rc.d/dhclient

@@ -49,6 +49,11 @@ dhclient_prestart()
 		rc_flags="${rc_flags} -b"
 	fi
 
+
+	# /var/run/dhclient is not guaranteed to exist,
+	# e.g. if /var/run is a tmpfs
+	install -d -o root -g wheel -m 755 ${pidfile%/*}
+
 	rc_flags="${rc_flags} ${ifn}"
 }