Fix up handling of C/POSIX collations.

Fix up handling of C/POSIX collations.

Install just one instance of the "C" and "POSIX" collations into
pg_collation, rather than one per encoding.  Make these instances exist
and do something useful even in machines without locale_t support: to wit,
it's now possible to force comparisons and case-folding functions to use C
locale in an otherwise non-C database, whether or not the platform has
support for using any additional collations.

Fix up severely broken upper/lower/initcap functions, too: the C/POSIX
fastpath now does what it is supposed to, and non-default collations are
handled correctly in single-byte database encodings.

Merge the two separate collation hashtables that were being maintained in
pg_locale.c, and be more wary of the possibility that we fail partway
through filling a cache entry.