pgsql: Fix regex_fixed_prefix() to cope reasonably well with regex

Log Message:
-----------
Fix regex_fixed_prefix() to cope reasonably well with regex patterns of the
form '^(foo)$'. Before, these could never be optimized into indexscans.
The recent changes to make psql and pg_dump generate such patterns (for \d
commands and -t and related switches, respectively) therefore represented
a big performance hit for people with large pg_class catalogs, as seen in
recent gripe from Erik Jones. While at it, be more paranoid about
case-sensitivity checking in multibyte encodings, and fix some other
corner cases in which a regex might be interpreted too liberally.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
regexp.c (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/regexp.c.diff?r1=1.66&r2=1.67)
selfuncs.c (r1.216 -> r1.217)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.216&r2=1.217)
pgsql/src/include/utils:
builtins.h (r1.283 -> r1.284)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h.diff?r1=1.283&r2=1.284)