eventchn: 'fd' and 'pending' are internal details of the Activations module

Move these Unix userspace specific details into the Unix userspace
specific Activations module. The xen version in mirage-platform can do
this differently.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
event: merge the OCaml implementation with Mirage

All the generation id logic for resume is here (even if it's not
necessary for Unix userspace)

All the stubs have the same name as their Mirage counterparts following
the naming convention: stub_evtchn_foo (the standard abbreviation of
"event channel" is "evtchn" not "eventchn")

Signed-off-by: David Scott <dave.scott@eu.citrix.com>