Review: alternate psql file locations

Reference: http://archives.postgresql.org/message-id/4F00CA9E.9000307@dunslane.net

This patch adds support for two new environment variables:

PSQL_HISTORY: Alternative location for the command history file.
PSQLRC: Alternative location of the user's .psqlrc file.

The context diff patch applies cleanly with a few offsets. Builds normally and all tests pass, although it adds no new tests. The documentation looks fine. The functionality is useful and works perfectly well in my quick test.

Marking ready for committer.

Best,

David