[2/2] imagehlp: Implement ImageGetDigestStream

[Dan Kegel]

I was worried that mapping the file will fail if you're
low on address space and are running this function
on a large PE file.
It ought to be possible to write a test to see how
the function behaves when you've used up oodles
of address space, though it'd be tricky to make
the test reliable.

I see that Windows has a similar problem in MapFileAndCheckSum
(which is said to be a caller of this function)
http://www.daniweb.com/forums/thread122484.html
so who knows, maybe Windows also maps the file...