WAL file rotation/ XLog Switch

Include here a prototype patch that implements pg_switch_xlog() in line
with earlier discussions about how this should be implemented.

This patch implements
- separate function for manual xlog switch
- internals to allow pg_stop_backup() to perform auto log switching

Patch applies cleanly to cvstip, passes make check and also switches
xlogs cleanly. I've not tested recovery yet, though will do so shortly
--- yes, of course that is critical, before you say so.

I'm shipping this very early to allow discussion, rather than test it
fully and leave people wondering what it looks like.

A production version will be ready prior to 8.2 code freeze.

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