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
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/9042f583429ef880665dd36b7bcad45ba62dea7b

Modified Files
--------------
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(-)