Hand controlling TTY to child, if we have one

[chriskuehl]

Fixes #51

This uses the patch from @Ehlers almost verbatim, plus adds a test.

The new behavior is that dumb-init will hand the controlling TTY to its child, if it has one. This fixes the fairly common case where running docker run -ti <image> dumb-init bash produces:

bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell
root@a75d2eac361b:/# 

With this change, you get job control in the shell (and no warnings printed). The test checks exactly this (outside of Docker, though).

[ghost]

I'd like to propose some changes to the ioctl in my patch:

The ioctl are using not the correct arguments, see http://man7.org/linux/man-pages/man4/tty_ioctl.4.html, TIOCSCTTY has an int argument and TIOCNOTTY has none. Furthermore I suggest using STDIN_FILENO instead of 0 for the file descriptor.

So I propose to change ioctl(0, TIOCNOTTY, NULL) to ioctl(STDIN_FILENO, TIOCNOTTY) and ioctl(0, TIOCSCTTY, NULL) to ioctl(STDIN_FILENO, TIOCSCTTY, 0).

[chriskuehl]

@Ehlers thanks for taking a look! I added a commit with those changes. Does it look right now?

[ghost]

Look good for me (and for circleci :-)