Implement optional syslog logging using ordinary

bb_xx_msg calls, and convert networking/* to it.
The rest of bbox will be converted gradually.

libbb/error_msg.c

@@ -18,7 +18,6 @@ void bb_error_msg(const char *s, ...)
 	va_list p;
 
 	va_start(p, s);
-	bb_verror_msg(s, p);
+	bb_verror_msg(s, p, NULL);
 	va_end(p);
-	putc('\n', stderr);
 }