lxc-attach: add comment, & implement login_tty()
lxc-attach: add comment, & implement login_tty()

- The code required to prepare an fd to act as a login tty is shared among
  pty_on_host_callback() and fork_pty(). This implements login_pty(), a
  minimalistic login_tty() clone, to avoid code redundancy.
- Give pty_in_container() a slightly extended comment.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@mailbox.org>

ysbnim/lxc,aeris/lxc,hfuCN/lxc,thmo/lxc,GreatFruitOmsk/lxc,tych0/lxc,akshaykarle/lxc,LynxChaus/lxc,aeris/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,cjwatson/lxc,aeris/lxc,terceiro/lxc,hfuCN/lxc,akshaykarle/lxc,tych0/lxc,GreatFruitOmsk/lxc,QingweiPeterLan/cs188-lxc,federicobriata/lxc,akshaykarle/lxc,federicobriata/lxc,LynxChaus/lxc,ysbnim/lxc,cjwatson/lxc,hfuCN/lxc,ysbnim/lxc,thmo/lxc,hallyn/lxc,hallyn/lxc,caglar10ur/lxc,terceiro/lxc,caglar10ur/lxc,LynxChaus/lxc,GreatFruitOmsk/lxc,QingweiPeterLan/cs188-lxc,ss1h2a3tw/lxc,federicobriata/lxc,hallyn/lxc,thmo/lxc,caglar10ur/lxc,tych0/lxc,QingweiPeterLan/cs188-lxc,terceiro/lxc,cjwatson/lxc