Ensure libmd symbols do not clash with libcrypto

Add a prefix to all symbols in libmd to avoid incompatibilites
with same-named, but not binary compatible, symbols from libcrypto.

Also introduce Weak aliases to avoid the need to rebuild dependent
binaries and a major version bump.

PR: 199119
Differential Revision: D2216
Reviewed by: roberto, delphij
MFC after: 2 weeks