User control over psql error stream

Hi,

This patch gives the end user control over psql's
error stream.  This allows a single psql session
to use \o to send both errors and table output
to multiple files.  Useful when capturing test output, etc.

Control is provided via a new "estream" \pset.  Here's
the docs.

-------------------------<snip>------------------------
estream

    Controls the output stream(s) used to report error messages. Value
must be one of: stderr (the default), which sends errors to the
standard error stream; query, which injects error messages into the
query result output stream; or both, which sends errors to both output
streams. "Error messages" are comprised of errors from psql and notice
messages and errors from the database server.
-------------------------<snip>------------------------

Against head.

psql-estream.patch       The patch.
psql-estream_test.patch  Adds a regression test to test the patch.

There's a number of problems with psql-estream_test.patch,
the most notable is that it probably won't work on
MS Windows because it uses /dev/null to avoid touching the
host filesystem.   I'm not sure whether this should have
a regression test and if so what the right way is to do it.

Note that psql-stream.patch includes some re-writing of
the docs for the psql \o option that goes slightly beyond
the minimum change required to explain \pset estream's effects.

Regards,

Karl <kop@meme.com>
Free Software:  "You don't pay back, you pay forward."
                 -- Robert A. Heinlein