pipe chunks protocol

This patch implements the protocol Tom suggested for writing to the
syslogger pipe. It seems to pass my tests (basically "make installcheck"
against a server with stderr redirection turned on and log_statement set
to 'all').

The effect of this should be to prevent two problems:
. partial messages get written to the log file, which messes with
rotation, and
. messages from various backends get interleaved, causing garbled logs.

Please review ASAP. I want to get this applied soon so that a) it gets
wider testing and b) I can use it as the basis for the adapted CSV log
patch. If this is acceptable I intend to backpatch this all the way to
wherever we started using the syslogger pipe (was that 8.0?).

cheers

andrew