Re-implement extraction of fixed prefixes from regular expressions.

To generate btree-indexable conditions from regex WHERE conditions (such as
WHERE indexed_col ~ '^foo'), we need to be able to identify any fixed
prefix that a regex might have; that is, find any string that must be a
prefix of all strings satisfying the regex.  We used to do that with
entirely ad-hoc code that looked at the source text of the regex.  It
didn't know very much about regex syntax, which mostly meant that it would
fail to identify some optimizable cases; but Viktor Rosenfeld reported that
it would produce actively wrong answers for quantified parenthesized
subexpressions, such as '^(foo)?bar'.  Rather than trying to extend the
ad-hoc code to cover this, let's get rid of it altogether in favor of
identifying prefixes by examining the compiled form of a regex.

To do this, I've added a new entry point "pg_regprefix" to the regex library;
hopefully it is defined in a sufficiently general fashion that it can remain
in the library when/if that code gets split out as a standalone project.

Since this bug has been there for a very long time, this fix needs to get
back-patched.  However it depends on some other recent commits (particularly
the addition of wchar-to-database-encoding conversion), so I'll commit this
separately and then go to work on back-porting the necessary fixes.

src/backend/regex/README

@@ -7,12 +7,13 @@ So this file is an attempt to reverse-engineer some docs.
 General source-file layout
 --------------------------
 
-There are four separately-compilable source files, each exposing exactly
+There are five separately-compilable source files, each exposing exactly
 one exported function:
 	regcomp.c: pg_regcomp
 	regexec.c: pg_regexec
 	regerror.c: pg_regerror
 	regfree.c: pg_regfree
+	regprefix.c: pg_regprefix
 (The pg_ prefixes were added by the Postgres project to distinguish this
 library version from any similar one that might be present on a particular
 system.  They'd need to be removed or replaced in any standalone version
@@ -44,6 +45,7 @@ regexec.c		Top-level regex execution code
 rege_dfa.c		DFA creation and execution
 regerror.c		pg_regerror: generate text for a regex error code
 regfree.c		pg_regfree: API to free a no-longer-needed regex_t
+regprefix.c		Code for extracting a common prefix from a regex_t
 
 The locale-specific code is concerned primarily with case-folding and with
 expanding locale-specific character classes, such as [[:alnum:]].  It