adding GUC variable xrefs

I'm planning to replace most of the <varname> tags enclosing GUC
variables in the docs with <xref> tags that allow the user to directly
jump to the documentation for that particular configuration variable
(that also requires adding "id" and "xreflabel" attributes to the GUC
var documentation). The "xreflabel" of a GUC var is just it's name, so
the actual text that is generated should be the same (the presentation
will be a little different though).

This is part of a general project to add more cross-references to our
documentation, so that we take better advantage of the hypertext
nature of HTML (and PDF, although I'm not sure if the PDF versions of
our docs support links -- in theory they could...)

Unless anyone objects, I'll start making the necessary changes soon.
An example of the kind of change I'm planning is attached as a patch.

-Neil