add EPOLLEXCLUSIVE epoll flag from linux v4.5

new flag for exclusive wakeup mode when an event source fd is attached
to multiple epoll fds but they should not all receive the events.
new in linux commit df0108c5da561c66c333bb46bfe3c1fc65905898

fix epoll structure alignment on non-x86_64 archs

this fix is far from ideal and breaks the rule of not using
arch-specific #ifdefs, but for now we just need a solution to the
existing breakage.

the underlying problem is that the kernel folks made a very stupid
decision to make misalignment of this struct part of the kernel
API/ABI for x86_64, in order to avoid writing a few extra lines of
code to handle both 32- and 64-bit userspace on 64-bit kernels. I had
just added the packed attribute unconditionally thinking it was
harmless on 32-bit archs, but non-x86 32-bit archs have 8-byte
alignment on 64-bit types.

add EPOLLWAKEUP flag to sys/epoll.h

align EPOLL_* flags with fcntl O_* flag definitions, which vary by arch

the old definitions were wrong on some archs. actually, EPOLL_NONBLOCK
probably should not even be defined; it is not accepted by the kernel
and it's not clear to me whether it has any use at all, even if it did
work. this issue should be revisited at some point, but I'm leaving it
in place for now in case some applications reference it.

initial check-in, version 0.5.0