lib/vty: add vty_stdio at-close hook

lib/vty: add vty_stdio at-close hook

This is intended to be used for either "exit on close", "fork on close"
or "reopen vty on close" functionality for the stdio vty.  Which of
these options to take depends on the context, the use case right now is
test programs exiting on EOF.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 464ccf36b4aa1b942cad413ea30267b4bf9e6315)