Add `io::ErrorKind::TooManyOpenFiles` Adds an unstable `io::ErrorKind::TooManyOpenFiles` for the open-file-limit condition. `EMFILE` and `ENFILE` currently decode to `ErrorKind::Uncategorized`, so stable code cannot tell that an operation failed because the process or the system ran out of file descriptors without inspecting `raw_os_error()` and a platform-specific `libc`/`windows-sys` constant. Implements the accepted ACP rust-lang/libs-team#818, including its decision to collapse `EMFILE` and `ENFILE` into a single variant. Finer-grained handling stays available through `raw_os_error()`. The variant maps: - `EMFILE` / `ENFILE` on Unix and WASI - `ERROR_TOO_MANY_OPEN_FILES` / `WSAEMFILE` on Windows - `FR_TOO_MANY_OPEN_FILES` on VEXos Tracking issue: rust-lang/rust#158319 r? libs