add paging to "show msgbuf"

[Scott Long]

Doug White wrote:
> 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.
> 

Looks good!

Scott