libshared: move md5 to BPrivate namespace

Otherwise it clashes with the implementation in OpenSSL which uses the
same names but now has a different ABI.

Change-Id: I5cb3ff97d7b28de978cdcbd8a06f25f65fb53784
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2854
Reviewed-by: Kyle Ambroff-Kao <kyle@ambroffkao.com>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

Remove GPL-licensed implementation of MD5

APE reader was using a GPL licensed version of MD5. A similar
implementation in the public domain was available in libnetapi, which I
moved to libshared so the APE reader can use it (and made some fixes,
missing const mainly). It only needs a small wrapper to use it easily
from C++ in a way compatible with the previous implementation.

Part of #13814.