Various fixes in the logic of XML functions:

- Add new SQL command SET XML OPTION (also available via regular GUC) to
  control the DOCUMENT vs. CONTENT option in implicit parsing and
  serialization operations.

- Subtle corrections in the handling of the standalone property in
  xmlroot().

- Allow xmlroot() to work on content fragments.

- Subtle corrections in the handling of the version property in
  xmlconcat().

- Code refactoring for producing XML declarations.