Add bound checks for ssl_min_protocol_version and ssl_max_protocol_version

Add bound checks for ssl_min_protocol_version and ssl_max_protocol_version

Mixing incorrect bounds in the SSL context leads to confusing error
messages generated by OpenSSL which are hard to act on.  New range
checks are added when both min/max parameters are loaded in the context
of a SSL reload to improve the error reporting.  Note that this does not
make use of the GUC hook machinery contrary to 41aadee, as there is no
way to ensure a consistent range check (except if there is a way one day
to define range types for GUC parameters?).  Hence, this patch applies
only to OpenSSL, and uses a logic similar to other parameters to trigger
an error when reloading the SSL context in a session.

Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20200114035420.GE1515@paquier.xyz