pgsql: Rearrange extension-related views as per recent discussion.

Rearrange extension-related views as per recent discussion.

The original design of pg_available_extensions did not consider the
possibility of version-specific control files.  Split it into two views:
pg_available_extensions shows information that is generic about an
extension, while pg_available_extension_versions shows all available
versions together with information that could be version-dependent.
Also, add an SRF pg_extension_update_paths() to assist in checking that
a collection of update scripts provide sane update path sequences.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/555353c0c59ada35ae59c8a76186e98d123fa8b3

Modified Files
--------------
doc/src/sgml/catalogs.sgml           |   90 +++++++-
doc/src/sgml/extend.sgml             |   14 ++
src/backend/catalog/system_views.sql |   14 +-
src/backend/commands/extension.c     |  409 ++++++++++++++++++++++++++++++++--
src/bin/psql/tab-complete.c          |    2 +-
src/include/catalog/catversion.h     |    2 +-
src/include/catalog/pg_proc.h        |    8 +-
src/include/utils/builtins.h         |    2 +
src/test/regress/expected/rules.out  |  125 ++++++-----
9 files changed, 570 insertions(+), 96 deletions(-)