pgsql: Rename PQsslAttributes() to PQsslAttributeNames(), and const-ify

Rename PQsslAttributes() to PQsslAttributeNames(), and const-ify fully.

Per discussion, the original name was a bit misleading, and
PQsslAttributeNames() seems more apropos. It's not quite too late to
change this in 9.5, so let's change it while we can.

Also, make sure that the pointer array is const, not only the pointed-to
strings.

Minor documentation wordsmithing while at it.

Lars Kanis, slight adjustments by me

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ab994cc00ec3e3700b2e62de9777d410fbb6ae84

Modified Files
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doc/src/sgml/libpq.sgml | 14 +++++++-------
doc/src/sgml/release-9.5.sgml | 28 ++++++++++++++++------------
src/interfaces/libpq/exports.txt | 2 +-
src/interfaces/libpq/fe-secure-openssl.c | 6 +++---
src/interfaces/libpq/fe-secure.c | 6 +++---
src/interfaces/libpq/libpq-fe.h | 2 +-
6 files changed, 31 insertions(+), 27 deletions(-)