Add tramp_hook_before_device_init hook, move hooks to their own header
Fix leaked file descriptor
Redirect STDIO of execvd' processes to /dev/null

* Fixes 30s freeze on dup2() in Android's init binary after it is
  started by trampoline if adb in trampoline is enabled
* I think the timeout is on /dev/console, which is wired
  to UART on hammerhead. dup2 tries to flush the file but
  UART driver won't accept immediately since nothing is connected
  to the UART lines.