Add routines for calculating SHA1 and MD5 digests, based on the

Add routines for calculating SHA1 and MD5 digests, based on the
gnutls package.  These routines are designed to replace the
current functions shahash() and md5sum(), but they are not
drop-in replacements because they have different signatures.

Prerequisites: packages gcrypt and gnutls.

This code is based on a contribution by Jason Stephenson.

A    include/opensrf/osrf_digest.h
A    src/libopensrf/osrf_digest.c

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1879 9efc2488-bf62-4759-914b-345cdb29e865