Rollup merge of #158870 - valentynkit:posix-io-error-single-file, r=joboet

std: merge the unix-like io::error modules into one file

`library/std/src/sys/io/error/` had a separate `decode_error_kind` (errno -> `io::ErrorKind`) for each POSIX-like target: `unix.rs`, `wasi.rs`, and `teeos.rs`. The three were near-identical hand copies and had drifted apart, each in its own way (rust-lang/rust#158490).

Per the discussion there, this folds `wasi.rs` and `teeos.rs` into `unix.rs`. `decode_error_kind` and `is_interrupted` are now shared; `errno`, `set_errno`, and `error_string` stay per-target behind `cfg`, next to the target-specific cases the file already had. A new mapping is written once and applies to all three.

The shared table is unix's existing one, so the merge also picks up the arms wasi and teeos were missing:

- wasi now maps `ENOTEMPTY` -> `DirectoryNotEmpty` (was `Uncategorized`).
- teeos now maps `EINPROGRESS` -> `InProgress`, `EMFILE`/`ENFILE` -> `TooManyOpenFiles`, and `EOPNOTSUPP` -> `Unsupported` (were `Uncategorized`).

All of those errnos are defined on the respective targets in libc, so the arms compile everywhere the file does. wasi's `EWOULDBLOCK` arm becomes the `x == EAGAIN || x == EWOULDBLOCK` guard the other two already use; that is a no-op on wasi, where `EWOULDBLOCK == EAGAIN`.

No test: with one shared `decode_error_kind` the copies can't drift again, and the decode arms are not tested today. (Let me know if it worth covering with tests)

Addresses rust-lang/rust#158490

r? @joboet