pgsql: Clean up installation directory choices for extensions.

Clean up installation directory choices for extensions.

Arrange for the control files to be in $SHAREDIR/extension not
$SHAREDIR/contrib, since we're generally trying to deprecate the term
"contrib" and this is a once-in-many-moons opportunity to get rid of it in
install paths. Fix PGXS to install the $EXTENSION file into that directory
no matter what MODULEDIR is set to; a nondefault MODULEDIR should only
affect the script and secondary extension files. Fix the control file
directory parameter to be interpreted relative to $SHAREDIR, to avoid a
surprising disconnect between how you specify that and what you set
MODULEDIR to.

Per discussion with David Wheeler.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/24d1280c4d75038f130495a25844c60d6810faab

Modified Files
--------------
doc/src/sgml/extend.sgml | 27 ++++++++++++++----------
doc/src/sgml/ref/create_extension.sgml | 2 +-
src/backend/commands/extension.c | 9 +++----
src/makefiles/pgxs.mk | 35 ++++++++++++++++++++++---------
4 files changed, 46 insertions(+), 27 deletions(-)