pgsql: Dodge a macro-name conflict with Perl.

Dodge a macro-name conflict with Perl.

Some versions of Perl export a macro named HS_KEY.  This creates a
conflict in contrib/hstore_plperl against hstore's macro of the same
name.  The most future-proof solution seems to be to rename our macro;
I chose HSTORE_KEY.  For consistency, rename HS_VAL and related macros
similarly.

Back-patch to 9.5.  contrib/hstore_plperl doesn't exist before that
so there is no need to worry about the conflict in older releases.

Per reports from Marco Atzeri and Mike Blackwell.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/68c1d7d42e553682f1d2723e623b6a3a4b02c75f

Modified Files
--------------
contrib/hstore/hstore.h                   |   10 +-
contrib/hstore/hstore_compat.c            |    2 +-
contrib/hstore/hstore_gin.c               |    8 +-
contrib/hstore/hstore_gist.c              |   16 ++--
contrib/hstore/hstore_io.c                |   83 ++++++++--------
contrib/hstore/hstore_op.c                |  148 +++++++++++++++--------------
contrib/hstore_plperl/hstore_plperl.c     |    7 +-
contrib/hstore_plpython/hstore_plpython.c |    8 +-
8 files changed, 153 insertions(+), 129 deletions(-)