pty: set XDG_VTNR if requested by caller

pty: set XDG_VTNR if requested by caller

A caller can now pass a VT-num to the PTY which will get set as XDG_VTNR
in the environment of new childs.

This can be used to tell systemd-logind to associate the session with
the correct VT. But note that this still cannot overwrite VT-associations
if the pty is created from within an existing session.

Reported-by: Thomas Hebb
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>