New string functions; initdb required

I've just committed changes which implement three SQL99 functions and
operators. OVERLAY() allows substituting a string into another string,
SIMILAR TO is an operator for pattern matching, and a new variant of
SUBSTRING() accepts a pattern to match.

Regression tests have been augmented and pass. Docs have been updated.
The system catalogs were updated, so it is initdb time. Details from the
cvs log below...

- Thomas

Implement SQL99 OVERLAY(). Allows substitution of a substring in a
string.
Implement SQL99 SIMILAR TO as a synonym for our existing operator "~".
Implement SQL99 regular expression SUBSTRING(string FROM pat FOR
escape).
Extend the definition to make the FOR clause optional.
Define textregexsubstr() to actually implement this feature.
Update the regression test to include these new string features.
All tests pass.
Rename the regular expression support routines from "pg95_xxx" to
"pg_xxx".
Define CREATE CHARACTER SET in the parser per SQL99. No implementation
yet.