Drop Log::Dispatch (finally) and simplify logging.

Kill global constants LOG_DEBUGLEVEL, SCREEN_DEBUGLEVEL, DEBUGCHAR,
INFOCHAR, INCOMINGCHAR, SAVECHAR, WARNCHAR, ERRORCHAR.

Kill global $logger and functions logsave, loginfo, logdebug, logwarn,
logerr.

(mylog): Kill first two parameters; all @_ are now catenated for the
log message.  Simply open $logfile and write the timestamp and
message, no more.

(logheader): Renamed from logincoming.  Adapt to new mylog.

(log_headers): s/logincoming/logheader/

(store_message): Call mylog instead of logsave.