Checksum libraries

[Anders F Björklund]

Since some people want to run MacPorts mirrors on Linux,
I re-added the support for using libcrypto for checksums.
There was also some breakage in the support for FreeBSD,
using an older (1999?) version of libmd, but that's fixed.

For Mac OS X, there are *no* changes. It will still use
CommonCrypto - for everything but the RIPEMD-160 digest...
When using FreeBSD or OpenSSL, all 4 digests use library
routines (that is: md5, sha1, rmd160, sha256) if possible.

http://trac.macports.org/changeset/77865

http://trac.macports.org/ticket/26813

--anders