Terminating pg_basebackup background streamer

If an error occurs in the foreground (backup) process of pg_basebackup, and
we exit in a controlled way, the background process (streaming xlog
process) would stay around and keep streaming.

This can happen for example if disk space runs out and there is very low
activity on the server. (If there is activity on the server, the background
streamer will also run out of disk space and exit)

Attached patch kills it off in disconnect_and_exit(), which seems like the
right thing to do to me.

Any objections to applying and backpatching that for the upcoming minor
releases?

Should we perhaps also consider adding a sigterm handler to pg_basebackup,
so if you send it a SIGTERM it kills off the background process as well?

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