add paging to "show msgbuf"

[Doug White]

This patch makes the "show msgbuf" ddb command use the pager instead of
just blasting it out. This helps me with a system that corrupts serial
output without flow control unless you give it bite-sized pieces.

http://people.freebsd.org/~dwhite/patches/subr_prf.c.20050602.patch

I'll commit this in a few days if there are no objections.

-- 
Doug White                    |  FreeBSD: The Power to Serve
dwhite at gumbysoft.com          |  www.FreeBSD.org