o Replace part-wise instructions for building world for jail(8) with

a simple make world; while this does a bit more work, it means that
jail(8) doesn't have to be kept in sync with /usr/src/Makefile{,.inc1}
which is a moving target.  MFC candidate.

Submitted by: FUJISHIMA Satsuki <sf@FreeBSD.org>
Reviewed by: phk
Also pointed out by: Phil Kernick <Phil@Kernick.org>