Log: No more argv parsing for log manager.

Earlier, the global setting for the syslog decided whether syslog
was enabled when skygw_logmanager_init was called, but not whether
logging to syslog actually was made.

Now syslog logging is enabled by default and the global setting
decides whether or not syslog logging actually is made. That is,
this opens up the possiblity for making it possible to turn on
and off sysloging at runtime.

Further, although the API led you to believe otherwise, it was
hardwired that LOGFILE_ERROR and LOGFILE_MESSAGE messages were
written to syslog.

The changed removed the need for passing an argv array explicitly.

log_manager/log_manager.h

@@ -153,8 +153,7 @@ int mxs_log_disable_priority(int priority);
 
 bool skygw_logmanager_init(const char* ident,
                            const char* logdir,
-                           log_target_t target,
-                           int argc, char* argv[]);
+                           log_target_t target);
 void skygw_logmanager_done(void);
 void skygw_logmanager_exit(void);