Rollup merge of #128316 - GrigorenkoPV:io_error_a_bit_more, r=dtolnay

Stabilize most of `io_error_more`

Sadly, venting my frustration with t-libs-api is not a constructive way to solve problems and get things done, so I will try to stick to stuff that actually matters here.

- Tracking issue for this feature was opened 3 years ago: #86442
- FCP to stabilize it was completed 19(!!) months ago: rust-lang/rust#86442 (comment)
- A PR with stabilization was similarly open for 19 months: #106375, but nothing ever came out of it. Presumably (it is hard to judge given the lack of communication) because a few of the variants still had some concerns voiced about them, even after the FCP.

So, to highlight a common sentiment:

> Maybe uncontroversial variants can be stabilised first and other variants (such as `QuotaExceeded` or `FilesystemLoop`) later? [^1]

[^1]: rust-lang/rust#106375 (comment)

> I would like to voice support stabilization of the uncontroversial variants. This would get those variants to stable and focus the discussion around the more controversial ones. I don't see any particular reason that all of these must be stabilized at the same time. [...] [^2]

[^2]: rust-lang/rust#106375 (comment)

> Maybe some less-controversial subset could be stabilized sooner? What’s blocking this issue from making progress? [^3]

[^3]: rust-lang/rust#86442 (comment) (got 30 upvotes btw) (and no response)

So this is exactly what this PR does. It stabilizes the non-controversial variants now, leaving just a few of them behind.

Namely, this PR stabilizes:

- `HostUnreachable`
- `NetworkUnreachable`
- `NetworkDown`
- `NotADirectory`
- `IsADirectory`
- `DirectoryNotEmpty`
- `ReadOnlyFilesystem`
- `StaleNetworkFileHandle`
- `StorageFull`
- `NotSeekable`
- `FileTooLarge`
- `ResourceBusy`
- `ExecutableFileBusy`
- `Deadlock`
- `TooManyLinks`
- `ArgumentListTooLong`
- `Unsupported`

This PR does not stabilize:
- `FilesystemLoop`
- `FilesystemQuotaExceeded`
- `CrossesDevices`
- `InvalidFilename`

Hopefully, this will allow us to move forward with this highly and long awaited addition to std, both allowing to still polish the less clear parts of it and not leading to stagnation.

r? joshtriplett
because they seem to be listed as a part of t-libs-api and were one of the most responsive persons previously