Move OpenSSL routines for min/max protocol setting to src/common/

Two routines have been added in OpenSSL 1.1.0 to set the protocol bounds
allowed within a given SSL context:
- SSL_CTX_set_min_proto_version
- SSL_CTX_set_max_proto_version

As Postgres supports OpenSSL down to 1.0.1 (as of HEAD), equivalent
replacements exist in the tree, which are only available for the
backend.  A follow-up patch is planned to add control of the SSL
protocol bounds for libpq, so move those routines to src/common/ so as
libpq can use them.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/4F246AE3-A7AE-471E-BD3D-C799D3748E03@yesql.se