integrate bsdiff into imgdiff, instead of using subprocess

Integrate the bsdiff code into imgdiff so we can just call it rather
than writing temp files and invoking bsdiff as a subprocess.  When
computing multiple patches from the same source file, we can also save
time by doing some bsdiff preprocessing on the source file only once.

(Note this renames the existing 'bsdiff.c' -- which actually does
patching in applypatch -- to 'bspatch.c', and introduces a new
bsdiff.c linked into imgdiff.)