vt, ssh: don't send interrupts on window resize

vt, ssh: don't send interrupts on window resize

When resizing windows, vt would signal ssh by updating
the window size and sending an interrupt. Ssh reacted
by forwarding both the winch and an interrupt.

This change adds a WINCH generation counter so that
ssh can differentiate between resizes and interrupts.
If an interrupt comes in, and the WINCH generation
changes, then the interrupt is taken as signalling a
WINCH.