pgsql: Create an explicit concept of collations that work for any encod

Create an explicit concept of collations that work for any encoding.

Use collencoding = -1 to represent such a collation in pg_collation.
We need this to make the "default" entry work sanely, and a later
patch will fix the C/POSIX entries to be represented this way instead
of duplicating them across all encodings.  All lookup operations now
search first for an entry that's database-encoding-specific, and then
for the same name with collencoding = -1.

Also some incidental code cleanup in collationcmds.c and pg_collation.c.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e3c732a85c0f247617b2d44ea567f35731b03ea6

Modified Files
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doc/src/sgml/catalogs.sgml                       |   12 ++--
src/backend/catalog/information_schema.sql       |    6 +-
src/backend/catalog/namespace.c                  |   48 +++++++++----
src/backend/catalog/pg_collation.c               |   67 +++++++++++-------
src/backend/commands/collationcmds.c             |   83 +++++++++++++++------
src/bin/psql/tab-complete.c                      |    2 +-
src/include/catalog/catversion.h                 |    2 +-
src/include/catalog/pg_collation.h               |   10 ++--
src/test/regress/expected/collate.linux.utf8.out |   12 ++--
9 files changed, 158 insertions(+), 84 deletions(-)