CVE-2016-10124: make lxc-attach use a pty

CVE-2016-10124: make lxc-attach use a pty

Previous versions of lxc-attach simply attached to the specified namespaces of
a container and ran a shell or the specified command without first allocating a
pseudo terminal. This made them vulnerable to input faking via a TIOCSTI ioctl
call after switching between userspace execution contexts with different
privilege levels. Newer versions of lxc-attach will try to allocate a pseudo
terminal master/slave pair on the host and attach any standard file descriptors
which refer to a terminal to the slave side of the pseudo terminal before
executing a shell or command. Note, that if none of the standard file
descriptors refer to a  terminal lxc-attach will not try to allocate a pseudo
terminal. Instead it will simply attach to the containers namespaces and run a
shell or the specified command.

(This is a backport of a series of patches fixing CVE-2016-10124.)

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>