For upstream/serial and logging fixes (#28)

* posix: logging: Use `stderr` instead of `stdout` for debug logging.

* posix: serial: Restore socket `termios` settings for `stdin` and `stdout` at exit.

src/cli/cli_serial.cpp

@@ -32,6 +32,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include <cli/cli.hpp>
@@ -101,6 +102,10 @@ void Serial::ReceiveTask(void)
 
             break;
 
+        case 3: // CTRL-C
+            exit(1);
+            break;
+
         case '\b':
         case 127:
             otPlatSerialSend(sEraseString, sizeof(sEraseString));