Disable custom baudrate support at runtime (Linux)

If picocom is compiled *with* custom-baudrate support (USE_CUSTOM_BAUD)
for Linux, then it uses a new set of ioctl's (TCGETS2 vs TCGETS, etc) to
access the serial ports. This patch allows the custom baudrate support
to be disabled at runtime (without recompiling), and picocom to switch
to using the old ioctl's.

To disable custom baudrate support (and switch back to the "old" ioctls)
simply define the environment variable NO_CUSTOM_BAUD, before starting
picocom.

This applies only to Linux and, obviously, only when picocom has been
compiled with custom baudrate support (USE_CUSTOM_BAUD).

picocom.c

@@ -1626,6 +1626,8 @@ show_usage(char *name)
 #endif
 #ifdef USE_CUSTOM_BAUD
     printf("  USE_CUSTOM_BAUD is enabled\n");
+    if ( ! use_custom_baud() )
+        printf("  NO_CUSTOM_BAUD is set\n");
 #endif
 
     printf("\nUsage is: %s [options] <tty port device>\n", s);