bb_info_msg: make it do atomic, unbuffered writes

udhcpc: fix a problem where we don't open listening socket fast enough

function                                             old     new   delta
bb_info_msg                                           70     126     +56
change_listen_mode                                    37      78     +41
udhcp_listen_socket                                  151     124     -27
udhcpc_main                                         2344    2285     -59

libbb/info_msg.c

@@ -12,6 +12,7 @@
 
 void FAST_FUNC bb_info_msg(const char *s, ...)
 {
+#ifdef THIS_ONE_DOESNT_DO_SINGLE_WRITE
 	va_list p;
 	/* va_copy is used because it is not portable
 	 * to use va_list p twice */
@@ -27,4 +28,29 @@ void FAST_FUNC bb_info_msg(const char *s, ...)
 		vsyslog(LOG_INFO, s, p2);
 	va_end(p2);
 	va_end(p);
+#else
+	int used;
+	char *msg;
+	va_list p;
+
+	if (logmode == 0)
+		return;
+
+	va_start(p, s);
+	used = vasprintf(&msg, s, p);
+	if (used < 0)
+		return;
+
+	if (ENABLE_FEATURE_SYSLOG && (logmode & LOGMODE_SYSLOG))
+		syslog(LOG_INFO, "%s", msg);
+	if (logmode & LOGMODE_STDIO) {
+		fflush(stdout);
+		/* used = strlen(msg); - must be true already */
+		msg[used++] = '\n';
+		full_write(STDOUT_FILENO, msg, used);
+	}
+
+	free(msg);
+	va_end(p);
+#endif
 }