amd64 minidump slowness

Hi,

At $JOB, we have machines with 400GB RAM that even the smallest
15GB amd64 minidump takes well over an hour. The major cause of
the slowness is that in minidumpsys(), blk_write() is called
PAGE_SIZE at a time. This causes blk_write() to poll the console
for the Ctrl-C abort once per page.

The attached patch changes blk_write() to be called with a run of
physically contiguous pages. This reduced the dump time by over a
magnitude. Of course, blk_write() could also be changed to poll
the console less frequently (like only on every IO).

If anybody else dumps on machines with lots of RAM, it would be
nice to know the difference this patch makes. I've got a second
set of patches that further reduces the dump time by over half that
I'll try to clean up soon.

http://people.freebsd.org/~bryanv/patches/minidump.patch