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.

src/backend/utils/misc/postgresql.conf.sample

@@ -406,6 +406,8 @@
 #default_transaction_read_only = off
 #statement_timeout = 0			# 0 is disabled
 #vacuum_freeze_min_age = 100000000
+#xmlbinary = 'base64'
+#xmloption = 'content'
 
 # - Locale and Formatting -