builtin-mailinfo.c infrastrcture changes

I am working on a project that required parsing through regular
mboxes that didn't necessarily have patches embedded in them.  I
started by creating my own modified copy of git-am and working
from there.  Very quickly, I noticed git-mailinfo wasn't able to
handle a big chunk of my email.

After hacking up numerous solutions and running into more
limitations, I decided it was just easier to rewrite a big chunk
of it.  The following patch has a bunch of fixes and features
that I needed in order for me do what I wanted.

Note: I'm didn't follow any email rfc papers but I don't think
any of the changes I did required much knowledge (besides the
boundary stuff).

List of major changes/fixes:
- can't create empty patch files fix
- empty patch files don't fail, this failure will come inside git-am
- multipart boundaries are now handled
- only output inbody headers if a patch exists otherwise assume those
headers are part of the reply and instead output the original headers
- decode and filter base64 patches correctly
- various other accidental fixes

I believe I didn't break any existing functionality or
compatibility (other than what I describe above, which is really
only the empty patch file).

I tested this through various mailing list archives and
everything seemed to parse correctly (a couple thousand emails).

[jc: squashed in another patch from Don's five patch series to
 fix the test case, as this patch exposes the bug in the test.]

Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>