git-am support for naked email messages (take 2)

git-am support for naked email messages (take 2)

This allows git-am to accept single-message files as well as mboxes.
Unlike the previous version, this one doesn't need to be explicitly told
which one it is; rather, it looks to see if the first line is a From
line and uses it to select mbox mode or not.

I moved the logic to do all this into git-mailsplit, which got a new
user interface as result, although the old interface is still available
for backwards compatibility.

[jc: applied with two obvious fixes.]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>