Replace OPENSSL_NO_SSL3_METHODs with dummies

SSLv3 has been deprecated since 2015 (and broken since 2014: "POODLE"); it
should not have shipped in FreeBSD 11 (2016) or 12 (2018). No one should use
it, and if they must, they can use some implementation outside of base.

There are three symbols removed with OPENSSL_NO_SSL3_METHOD:

SSLv3_client_method
SSLv3_method
SSLv3_server_method

These symbols exist to request an explicit SSLv3 connection to a server.
There is no good reason for an application to link or invoke these symbols
instead of TLS_method(), et al (née SSLv23_method, et al). Applications
that do so have broken cryptography.

Define these symbols for some pedantic definition of ABI stability, but
remove the functionality again (r361392) after r362620.

Reviewed by: gordon, jhb (earlier-but-equivalent version both)
Discussed with: bjk, kib
Differential Revision: https://reviews.freebsd.org/D25493