User guide: add error codes.

Learn how to use olinking correctly.
Add errcode.xml - the list of all error codes.
While at it, add markers to errcode.h signalling
that whenever it is changed, errcode.xml also needs a change.
Get rid of ERR_CODE_UNKNOWN_ERROR - it has no raison d'être.

Tweak the docbook to html XSL rules, in particular
table of contents generation.