Dynamic Tracing docs

Enclose new trace.sgml file as discussed on -docs.

This includes some heavily rewritten words and mostly verbatim programs
from http://blogs.sun.com/robertlor/entry/user_level_dtrace_probes_in,
as invited to do by Josh Berkus and Robert Lor on 27 Nov in private
correspondence prior to my post on -docs. There is substantial
additional material as well as genericisation so it is suitable for
inclusion in general docs.

This includes refactoring of some of the trace related GUC docs from
config.sgml, so its all in the one place.

SGML is clean apart from two xrefs that don't work properly:

openjade:config.sgml:4412:23:X: reference to non-existent ID
"TRACE-CONFIG-DEVELOPER"
openjade:wal.sgml:292:31:X: reference to non-existent ID "GUC-WAL-DEBUG"
openjade:/usr/share/sgml/docbook/dsssl-stylesheets/html/dblink.dsl:203:1:E: XRef LinkEnd to missing ID 'TRACE-CONFIG-DEVELOPER'
openjade:/usr/share/sgml/docbook/dsssl-stylesheets/html/dblink.dsl:203:1:E: XRef LinkEnd to missing ID 'GUC-WAL-DEBUG'

Quick fixes?

(I haven't proof-read the resulting .html as a result of the above
error, but seems like y'all would want to see this anyway).

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com