Add pg_current_logfile() function.

Add pg_current_logfile() function.

The syslogger will write out the current stderr and csvlog names, if
it's running and there are any, to a new file in the data directory
called "current_logfiles".  We take care to remove this file when it
might no longer be valid (but not at shutdown).  The function
pg_current_logfile() can be used to read the entries in the file.

Gilles Darold, reviewed and modified by Karl O.  Pinc, Michael
Paquier, and me.  Further review by Álvaro Herrera and Christoph Berg.