serial_logger: exit immediately upon SIGINT

It appears that `intHandler()` is designed to terminate the main loop
by setting the `_keepRunning` flag. However, this logic is defective,
as it assumes that the main loop will terminate within a reasonable
time. This is not the case:

    (gdb) start
    (gdb) continue
    <CTRL-C>
    (gdb) signal SIGINT
    Continuing with signal SIGINT.
    Notice:  Serial Log:Stopping!

    <CTRL-C>
    (gdb) backtrace
    0  0x00007ffff7cea121 in read () from /lib64/libc.so.6
    1  0x0000000000407002 in get_packet ()
    2  0x00000000004038b1 in _serial_logger ()
    3  0x00000000004026ed in main ()

The main loop gets stuck in a blocking call to `read()`. If there is
no data coming via the serial port, `read()` never returns, and the
porogram never exits.

This behavior is quite problematic, as, intuitively, CTRL-C
terminates most programs by sending SIGINT. To stop `serial_logger`,
one generally needs to freeze the program with CTRL-Z, and then
send SIGKILL to terminate it. I presume this is not the intended
behavior of the main loopp.

Fixing this behavior would require re-architecting the main loop to
use asynchronous or non-blocking calls. For the short term, simply
exit on SIGINT and SIGTERM.

source/serial_logger.c

@@ -100,8 +100,7 @@ void broadcast_log(char *msg){
 void intHandler(int dummy) {
   _keepRunning = false;
   LOG(SLOG_LOG, LOG_NOTICE, "Stopping!\n");
-  if (_playback_file)  // If we are reading file, loop is irevelent
-    exit(0);
+  exit(0);
 }
 #else
 int serial_logger (int rs_fd, char *port_name, int logLevel, int slogger_packets, char *slogger_ids)