Tighten up allowed names for custom GUC parameters.

Suppress unnecessary regex subre nodes in a couple more cases.

Improve memory management in regex compiler.

Allow complemented character class escapes within regex brackets.

Suppress compiler warning in new regex match-all detection code.

Avoid generating extra subre tree nodes for capturing parentheses.

Doc: remove src/backend/regex/re_syntax.n.

Change regex \D and \W shorthands to always match newlines.

Allow complemented character class escapes within regex brackets.