Add sha1.c and libsha1.h for doing SHA-1-based message-ID generation.

This code comes courtesy of Brian Gladman and Mikhail Gusarov.

Both files are available under the GPL and were downloaded as
version 0.2 of libsha1 from git://github.com/dottedmag/libsha1.git
with the following commit:

commit d0f0e7e0dc5ce2d58972cb5a492183c0d4e58433
Author: Mikhail Gusarov <dottedmag@dottedmag.net>
Date:   Mon Oct 20 22:38:47 2008 +0700

    Version bump.

    Signed-off-by: Mikhail Gusarov <dottedmag@dottedmag.net>