Support openssl replacements which don't export RAND_egd()

Summary: reSSL, for example, explicitly does not implement
RAND_egd() on the basis that:

"No software in 2014 should be using egd;
no software in 2014 should support using egd by accident."

Closes #4118

Reviewed By: @JoelMarcey

Differential Revision: D1656387

Signature: t1:1656387:1415077784:824f87d1b59db0bfd73a02f394a27baccd24cebd