Cleanup of the logging subsystem; allow logging to stderr only

Separated the logging logically from any remaining debugging
features. Introduced command line option for logging to stderr
only (for systemd and containers). The default log level is set
to (2) info.

Signed-off-by: Nikos Mavrogiannopoulos <n.mavrogiannopoulos@gmail.com>

tests/human_addr.c

@@ -25,6 +25,8 @@
 #include "../src/ip-util.h"
 #include "../src/ip-util.c"
 
+int syslog_open = 0;
+
 static unsigned ip_to_sockaddr(const char *ip, unsigned port, struct sockaddr_storage *ss)
 {
 	int ret;