This patch is an updated version of the lock listing patch. I've made
the following changes:

    - write documentation
    - wrap the SRF in a view called 'pg_locks': all user-level
      access should be done through this view
    - re-diff against latest CVS

One thing I chose not to do is adapt the SRF to use the anonymous
composite type code from Joe Conway. I'll probably do that eventually,
but I'm not really convinced it's a significantly cleaner way to
bootstrap SRF builtins than the method this patch uses (of course, it
has other uses...)

Neil Conway