mount: create a slave pty if it has to be bind-mounted to somewhere

Now we create pty pairs when file descriptors are restored.
The problem is that a slave tty can be bind-mounted to somewhere
and in this case we have to create this pair and hold a master file
descritore before related file descriptors will not be restored.

In this patch, a unix socket is used to hold file descriptros.
And we use SK_PEEK_OFF and MSG_PEEK to get any of them.

travis-ci: success for A few fixes to c/r a docker container with a console (rev3)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>