pg_basebackup for streaming base backups

This patch creates pg_basebackup in bin/, being a client program for
the streaming base backup feature.

I think it's more or less done now. I've again split it out of
pg_streamrecv, because it had very little shared code with that
(basically just the PQconnectdb() wrapper).

One thing I'm thinking about - right now the tool just takes -c
<conninfo> to connect to the database. Should it instead be taught to
take the connection parameters that for example pg_dump does - one for
each of host, port, user, password? (shouldn't be hard to do..)

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