Add support for ppoll()

[Susurrus]

poll() is already included, but ppoll() would be nice too. Since there's a PR for pselect() I figured ppoll() would be suitable for inclusion as well.

[kamalmarhubi]

@Susurrus it'd certainly be welcome! Any chance you'd be interested in adding it? I could walk you through it if so.

[Susurrus]

Yeah, I'd like to get this added, but it looks like I'll first need to add ppoll() to libc.

Also, what should the function definition be? Given the consensus in #516 I would like to do:
fn ppoll(fds: &mut [PollFd], timeout: Duration) -> Result<libc::c_int>. Does that work?

[Susurrus]

I can file an addition PR for #516 that will synchronize everything on using Duration, but I'm assuming the API discrepency will be acceptable in the short term?

[Susurrus]

Just filed a PR for this for libc, but it'll probably take a couple of revisions nailing down what platforms actually support this (rust-lang/libc#537).

[Susurrus]

Closed by 5d4967e