* Relax lifetime requirements for PollFd::new
Fixes#2118
* Take BorrowedFd as the argument for PollFd::new
&AsFd didn't work because there are 'static types, like std::fs::File,
which implement AsFd. The created BorrowedFd type within the
PollFd::new method would have a very brief lifetime, but the PhantomData
would capture the lifetime of the std::fs::File. Taking BorrowFd<'fd>
argument makes the lifetime explicit.
* fix legacy comment
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Co-authored-by: Steve Lau <stevelauc@outlook.com>