log_filename_prefix --> log_filename + strftime()

Attached is a patch which replaces the 'log_filename_prefix' configuration
directive with a similar 'log_filename' directive. It differs from the
former in the following ways:

+ allows embedded strftime() escapes ala Apache's rotatelogs;
+ eliminates hard-coded embedding of the postmaster pid;
+ makes the current hard-coded timestamp configurable;
+ changes the default log filename to exclude the PID;

This patch enables us to continue using our existing log-handling utilities
and filenaming conventions which we now use with Apache's rotatelogs.