lxc-attach: always allocate current controlling pty

lxc-attach: always allocate current controlling pty

lxc-attach uses lxc_console_create() to allocate a pty on the host.
lxc_console_create() in turn calls lxc_console_peer_default() which either
makes the current controlling pty our controlling pty for the container, or
uses whatever the user gave us (e.g. /dev/tty2 etc.). For lxc-attach we always
want the current controlling pty to be used. This commit ensures that we're in
fact always using the current controlling pty. The commit also fixes a segfault
when the user specified lxc.console.path = none.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>