md5: remove use of CommonCrypto-to-OpenSSL macros for the benefit of Leopard

For some reason, OS X 10.5's GCC suddenly stopped working correctly with
macros that change MD5_Init etc. in the code to CC_MD5_Init etc., so I
worked around this by removing use of the macros and inserting static
functions that just call CommonCrypto's implementations of the functions
instead.