metalink/md5: Use CommonCrypto on Apple operating systems

Previously the Metalink code used Apple's CommonCrypto library only if
curl was built using the --with-darwinssl option. Now we use CommonCrypto
on all Apple operating systems including Tiger or later, or iOS 5 or
later, so you don't need to build --with-darwinssl anymore. Also rolled
out this change to libcurl's md5 code.