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.

src/port/pgstrcasecmp.c

@@ -13,6 +13,10 @@
  *
  * NB: this code should match downcase_truncate_identifier() in scansup.c.
  *
+ * We also provide strict ASCII-only case conversion functions, which can
+ * be used to implement C/POSIX case folding semantics no matter what the
+ * C library thinks the locale is.
+ *
  *
  * Portions Copyright (c) 1996-2011, PostgreSQL Global Development Group
  *
@@ -123,3 +127,25 @@ pg_tolower(unsigned char ch)
 		ch = tolower(ch);
 	return ch;
 }
+
+/*
+ * Fold a character to upper case, following C/POSIX locale rules.
+ */
+unsigned char
+pg_ascii_toupper(unsigned char ch)
+{
+	if (ch >= 'a' && ch <= 'z')
+		ch += 'A' - 'a';
+	return ch;
+}
+
+/*
+ * Fold a character to lower case, following C/POSIX locale rules.
+ */
+unsigned char
+pg_ascii_tolower(unsigned char ch)
+{
+	if (ch >= 'A' && ch <= 'Z')
+		ch += 'a' - 'A';
+	return ch;
+}