Fix flag type names to match FOO_* => FooFlags convention

[kamalmarhubi]

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The convention was added in #287, and there are a few places that need changing. I'm going through all the bitflags as part of #315, and noticing them. Here's a (partial) list:

• [x] poll::EventFlags => PollFlags (1)
• [x] sys::epoll::EpollEventKind => EpollFlags (1)
• [x] sys::event::EventFlag => EvFlags
• [ ] sys::event::FilterFlag => NoteFlags
• [x] sys::eventfd::EventFdFlag => EFdFlags
• [x] sys::memfd::MemFdCreateFlag => MFdFlags
• [ ] sys::quota::QuotaValidFlags => QIFFlags (unsure about capitalization)
• [ ] sys::stat::{SFlag, Mode} => SFlags? (2)
• [ ] sys::statvfs::FsFlags => use sys::mount::MsFlags? (3)
• [x] sys::termios::{InputFlags, OutputFlags, ControlFlags, LocalFlags} => ? (4)

(1) these might need amending the convention. The flags are named POLLIN, POLLOUT, EPOLLIN, EPOLLOUT, etc: there is no underscore

(2) these are used in different parameters; needs to looked at a bit more carefully

(3) man page for statvfs(2) says

The field f_flag is a bit mask (of mount flags, see mount(8)).

(4) These flags have no common prefix.

[amanjeev]

I guess this can be closed as https://github.com/nix-rust/nix/pull/1024 is merged?

[asomers]

Some of the other types haven't been changed yet, though.

[amanjeev]

@asomers oh let me see which ones. Sorry I missed those. If you can point me to some other, I'd be glad to change those as well. It will give me some way to peruse through the source.

[palash25]

Hi :wave: I made a PR for sys::event can someone please review it.