[MEDIUM] introduce the "stats" keyword in global section

Removed old unused MODE_LOG and MODE_STATS, and replaced the "stats"
keyword in the global section. The new "stats" keyword in the global
section is used to create a UNIX socket on which the statistics will
be accessed.  The client must issue a "show stat\n" command in order
to get a CSV-formated output similar to the output on the HTTP socket
in CSV mode.
6 files changed