Refectoring of receivelog.c

I was working on adding the tar streaming functionality we talked about at
the developer meeting to pg_basebackup, and rapidly ran across the issue
that Andres has been complaining about for a while. The code in
receivelog.c just passes an insane number of parameters around. Adding or
changing even a small thing ends up touching a huge number of places.

Here's an attempt to refactor the code to instead pass around a control
structure. I think it's a definite win already now, and we can't just keep
adding new functionality on top of the current one.

I'll proceed to work on the actual functionality I was working on to go on
top of this separately, but would appreciate a review of this part
independently. It's mostly mechanical, but there may definitely be mistakes
- or thinkos in the whole idea...

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