add BUILD_HOST to pkg_summary

["Jeremy C. Reed" <reed%reedmedia.net@localhost>]

I patched info/show.c and lib/pkg_summary.5 (and version.h and catman 
file) for pkg_install to add BUILD_HOST.

Before I add it does anyone have anything against it?  Lots of the 
information is redundant. I am most interested in the hostname it 
provides.  (It is already available in +BUILD_INFO, but I don't want to 
fetch and extract that from each package.)