New "exists" feature, lldb bug fixes.

Thanks to our contributors @urklc and @dot-asm!

from @urklc:

• (-e, --exists) check if the app with given bundle_id is installed or not

from @dot-asm:

• allow to pass command line arguments from (lldb) prompt;
• make lldb exit as opposite to relying on kill (which is problematics
  because it messes up tty settings);
• handle event timeout in autoexit_command to avoid endless loop;
• use dynamic TCP port and listen on localhost (no need to expose the
  port to Internet);
• close accept socket once connection is established;