pgsql: Simplify code related to compilation of SSL and OpenSSL

Simplify code related to compilation of SSL and OpenSSL

This commit makes more generic some comments and code related to the
compilation with OpenSSL and SSL in general to ease the addition of more
SSL implementations in the future.  In libpq, some OpenSSL-only code is
moved under USE_OPENSSL and not USE_SSL.

While on it, make a comment more consistent in libpq-fe.h.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/5382CB4A-9CF3-4145-BA46-C802615935E0@yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/092b785fad3de3f81355a4b2420aa39a1bc0ccd5

Modified Files
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src/backend/libpq/hba.c          | 2 +-
src/include/libpq/libpq-be.h     | 2 +-
src/include/pg_config_manual.h   | 3 +--
src/interfaces/libpq/fe-secure.c | 9 ++++++++-
src/interfaces/libpq/libpq-fe.h  | 2 +-
5 files changed, 12 insertions(+), 6 deletions(-)