unix: fix EPOLLPRI conflicting with READABLE
unix: fix EPOLLPRI conflicting with READABLE

On Linux,
Ready::from(UnixReady::priority()).is_writable() = true

Use a separate bit instead.

Fixes:
https://github.com/carllerche/mio/issues/896

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@redhat.com>

carllerche/mio,murarth/mio,carllerche/mio