WIP: Separate log file for extension

Attached is a draft patch to allow extension to write log messages to a
separate file. It introduces a concept of a "log stream". The extension's
shared library gets its stream assigned by calling this function from
_PG_init()

my_stream_id = get_log_stream("my_extension", &my_log_stream);

Then it's supposed to change some of its attributes

adjust_log_stream_attr(&stream->filename, "my_extension.log");

and to use the stream id in ereport() calls

ereport(LOG, (errmsg("Hello world"), errstream(my_stream_id)));

The EXEC_BACKEND mechanism makes initialization of the log streams by
postmaster child processes non-trivial. I decided to extend
save_backend_variables() and restore_backend_variables() accordingly. Maybe
someone has better idea.

pgaudit seems to be the most obvious use case for this enhancement, but it
might be useful for many other extensions.

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at