on win32/apple reading packets from the interface does not count as an io operation.
manually trigger pump on win32/apple to pretend that it is an io event.
add platform quark function MaybeWakeUpperLayers on vpn::Interface to manaully wake up the other components on platforms that need that (ones on which packet io is not done via io events).
on non linux platforms, use uv_prepare_t instead of uv_check_t as the former triggers before blocking for io, instead of after. this better matches linux's order of operations in libuv.
When we enable/disable exit mode on this restarts the unbound DNS
responder with the DNS trampoline (or restores upstream, when disabling)
to properly route DNS requests through the tunnel (because libunbound's
direct requests don't get tunneled because unbound is inside the network
extension).
