minidump size on amd64

[Andriy Gapon]

So, here is the next version of the patch:
http://people.freebsd.org/~avg/amd64-minidump.4.diff

Changes since the last version:
1. libkvm - try to support both the new and the previous formats/versions of
amd64 minidump.  I am not entirely sure about style in which I handled handling
of version 1 minidump.  Identifier names like pmapsize (for "page map size") and
page_map could also be improved, perhaps.
2. kernel - implemented dumping of 1GB pages via "fake" 512 x 2MB pages per
Alan's suggestion.

The change is only compile tested so far.  Not sure if it's possible to test
handling 1GB pages yet :-)

As always, reviews, testing and suggestions are very welcome.
Thank you for your help!
-- 
Andriy Gapon