Change of vterm handling logic on backend chardev input event (#40)

Change of vterm handling logic on backend chardev input event (#40)

* fix: term should not ignore underlying input event

* term should always respond the input event from underlying
  termport awared character device, by process the raw input
  buffer according one's lflags.ICANNO setting when the data
  is avaliable

* fix: race when parent get destroyed before children

* fix: orphan a process's children when it is about to destroy
* fix: devzero: mis-used offset cause segfault

* add support of setting serial io speed

* fix: truncate the overflowed input instead of running over.

* rewrite the line control

* rewrite the line control implementation to make it
  clean and less hacky.
* correct some POSIX compliance issue related to some
  control characters.
* fix issue where ANSI line control failed to echo the
  transformed special character
* add a sanity filter to sh to filter out any unprintable
  character.

* fix: make lxconsole work with vterm refactor

* fix: tdev_do_read do not check the linebuffer::sflag

* add a config term for lxconsole (vgacon)

* rename tty/tty.c to vga_rawtty.c for clarity