Add O_ASYNC/FASYNC to OFlags (#1312)

Closes #1243

Author: sylfn

src/backend/libc/fs/types.rs

@@ -338,6 +338,20 @@ bitflags! {
         #[cfg(any(linux_kernel, solarish))]
         const LARGEFILE = bitcast!(c::O_LARGEFILE);
 
+        /// `O_ASYNC`, `FASYNC`
+        ///
+        /// Note that this flag can't be used with [`rustix::fs::open`] family of functions, use
+        /// [`rustix::fs::fcntl_setfl`] instead
+        #[cfg(not(any(
+            target_os = "aix",
+            target_os = "espidf",
+            target_os = "haiku",
+            target_os = "wasi",
+            target_os = "vita",
+            solarish
+        )))]
+        const ASYNC = bitcast!(c::O_ASYNC);
+
         /// <https://docs.rs/bitflags/*/bitflags/#externally-defined-flags>
         const _ = !0;
     }

src/backend/linux_raw/fs/types.rs

@@ -257,6 +257,12 @@ bitflags! {
         /// It will may be reported in return of `fcntl_getfl`, though.
         const LARGEFILE = linux_raw_sys::general::O_LARGEFILE;
 
+        /// `O_ASYNC`, `FASYNC`
+        ///
+        /// Note that this flag can't be used with [`rustix::fs::open`] family of functions, use
+        /// [`rustix::fs::fcntl_setfl`] instead
+        const ASYNC = linux_raw_sys::general::FASYNC;
+
         /// <https://docs.rs/bitflags/*/bitflags/#externally-defined-flags>
         const _ = !0;
     }