Rename OFlag/AtFlag to OFlags/AtFlags. (#1951)

* Rename `OFlag`/`AtFlag` to `OFlags`/`AtFlags`.

This makes them consistent with `PollFlags` and common usage of
bitflags types in Rust code in general.

POSIX does tend to use names like `oflag` and `flag`, so this is in mild
disagreement with POSIX style, however I find this particular aspects of
POSIX confusing because these values hold multiple flags.

* rustfmt

crates/wasi-common/src/sys/unix/emscripten/mod.rs

@@ -3,4 +3,4 @@ pub(crate) mod osdir;
 #[path = "../linux/path.rs"]
 pub(crate) mod path;
 
-pub(crate) const O_RSYNC: yanix::file::OFlag = yanix::file::OFlag::RSYNC;
+pub(crate) const O_RSYNC: yanix::file::OFlags = yanix::file::OFlags::RSYNC;