Re: fix bgworkers in EXEC_BACKEND

I committed this with minor tweaks to avoid having to scan the
registered workers list on each registration.  Opinions on this error
report are still welcome:

> +               ereport(LOG,
> +                               (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
> +                                errmsg("too many background workers"),
> +                                errdetail("Up to %d background workers can be registered with the current settings.",
> +                                                  MAX_BACKENDS - (MaxConnections + autovacuum_max_workers + 1))));

Thanks to everyone for their input --- and happy 2013 hacking to all.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services