Explicit psqlrc

I've now for the second time found myself wanting to specify an
explicit psqlrc file instead of just parsing ~/.psqlrc, so attached is
a patch that accepts the PSQLRC environment variable to control which
psqlrc file is used.

Any objections to this (obviously including documentation - this is
just the trivial code)

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/