network, sockets: Replace `POLL_IN`, `POLL_OUT`, etc. constants with an `enum class PollEvents`

Actually, two enum classes, since for some reason there are two separate
yet identical `PollFD` types used in the codebase.  I get that one is
ABI-compatible with the Switch while the other is an abstract type used
for the host, but why not use `WSAPOLLFD` directly for the latter?

Anyway, why make this change?  Because on Apple platforms, `POLL_IN`,
`POLL_OUT`, etc. (with an underscore) are defined as macros in
<sys/signal.h>.  (This is inherited from FreeBSD.)  So defining
a variable with the same name causes a compile error.

I could just rename the variables, but while I was at it I thought I
might as well switch to an enum for stronger typing.

Also, change the type used for values copied directly to/from the
`events` and `revents` fields of the host *native*
`pollfd`/`WSASPOLLFD`, from `u32` to `short`, as `short` is the correct
canonical type on both Unix and Windows.