Avoid collisions with function names in openssl headers.

Avoid collisions with function names in openssl headers.

Just using MD5, SHA1, RMD160 and SHA256 for defines collides with
functions of the same name in OpenSSL. This can cause compilation
issues in downstream consumers if they use OpenSSL for the hash
functions instead of libmd.

Reviewed by:	sjg
Obtained from:	Juniper Networks, Inc.
Differential Revision:	https://reviews.freebsd.org/D26321