observe the xmloption.

Reorganize the representation of the XML option in the parse tree and the
API to make it easier to manage and understand.

Add regression tests for parsing back XML expressions.

classified the conditions under which this is safe to do (see source
code comment).

- 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.

libxml as the detail message.

As per <http://archives.postgresql.org/pgsql-hackers/2006-12/msg01087.php>.

For converting error codes to messages, we only need to cover those codes
that we raise ourselves now.

function xmlagg.

with new GUC parameter "xmlbinary" that controls the output encoding, as
per SQL/XML standard.

declarations are ignored and removed, in binary mode they are honored as
specified by the XML standard.

coercion to type xml was a mistake.  Escape values so they are valid
XML character data.

our own printing dance.  This does a better job of quoting and escaping the
values.

- correct error codes
- do syntax checks in correct order
- strip leading spaces of argument

hand-crafted parser for the XML declaration, because libxml doesn't seem
to allow this.

sets the items, and serializes the value back (rather than adding an
arbitrary number of XML preambles as before).

The libxml memory management via palloc had to be disabled because it
crashes when libxml tries to access memory that was helpfully freed
earlier by PostgreSQL.  This needs further thought.

back-stamped for this.

Use a TRY block instead of (inadequate) ad-hoc coding to ensure that
libxml is cleaned up after a failure.  Report the intended SQLCODE
instead of defaulting to XX000.  Avoid risking use of a dangling
pointer by keeping the persistent error buffer in TopMemoryContext.
Be less trusting that error messages don't contain %.

This patch doesn't do anything about changing the way the messages
are put together --- this is just about mechanism.

bletcherous and unsafe manipulation of global encoding setting.
Clean up libxml reporting mechanism a bit (it still looks like a
dangling-pointer crash waiting to happen, though, not to mention
being far less than sane from a localization standpoint).

the XmlExpr code in various lists, use a representation that has some hope
of reverse-listing correctly (though it's still a de-escaping function
shy of correctness), generally try to make it look more like Postgres
coding conventions.