feat(cli): add 'pwn errno --list' + perror-string lookup (#2657)

[ChrisJr404]

Closes #2657.

What

`pwn errno` previously supported looking up a value either by integer or by errno macro name. The issue (filed by @k4lizen) asks for the inverse direction: "I see a perror message in the wild, what errno does it map to?" — e.g. `Cannot allocate memory` → `ENOMEM`.

Three small additions to the existing CLI:

1. `-l / --list` — dump every errno code visible to the standard library, one per line: ` `. The `--list` form was the explicit suggestion in the discussion.
2. `-s / --search ` — force a case-insensitive substring match against `os.strerror(N)` for all errnos and print every match.
3. Bare `pwn errno ` — when the argument is neither an integer nor a known errno macro, automatically fall back to the same substring search. So `pwn errno "Cannot allocate memory"` Just Works.

Output for the integer / name path is unchanged.

Demo

```
$ pwn errno 13
#define EACCES 13
Permission denied

$ pwn errno EACCES
#define EACCES 13
Permission denied

$ pwn errno "Cannot allocate memory"
#define ENOMEM 12
Cannot allocate memory

$ pwn errno -s broken
#define EPIPE 32
Broken pipe

$ pwn errno --list | head -3
1 EPERM Operation not permitted
2 ENOENT No such file or directory
3 ESRCH No such process
```

Why

The use case from the issue:

I want stuff like `pwn errno "Cannot allocate memory"` to return EACCES. Can often come in handy when I see the message being printed via errno and want to check the man pages or source to see what condition causes that.

`/usr/bin/errno -l | grep ...` works on Linux but isn't always installed; `pwn errno` ships with pwntools.

Tests

Verified against the local installation:

```
$ pwn errno 13 -> EACCES / Permission denied
$ pwn errno EACCES -> EACCES / Permission denied
$ pwn errno "Cannot allocate memory" -> ENOMEM
$ pwn errno -s broken -> EPIPE
$ pwn errno --list | head -> sorted dump of all errnos
$ pwn errno -> argparse error: 'error required (or pass --list)'
$ pwn errno NotARealError -> No errno for NotARealError
```

Notes

• Pure-additive: no existing invocation form changes its output.
• The positional `error` is now `nargs='?'` so `--list` can run without it; argparse emits the missing-arg error otherwise.
• The substring fallback is only triggered when the input doesn't parse as an int or errno name — there's no risk of mis-classifying a known macro like `EACCES` as a search term.

[Arusekk]

I can now see that the original code did not use architecture-specific constants, but it definitely should.
See the -c/--context option from tools like asm/disasm. As an example, ECONNRESET can be different depending on architecture:

$ grep ECONNRESET pwnlib/data/includes/generator/linux/diet/errno.h
#define ECONNRESET      54      /* Connection reset by peer */
#define ECONNRESET      131     /* Connection reset by peer */
#define ECONNRESET      54      /* Connection reset by peer */
#define ECONNRESET      232     /* Connection reset by peer */
#define ECONNRESET      104     /* Connection reset by peer */

Also, @ChrisJr404 do you use some kind of a template for your PRs, or generated descriptions? They are a little oddly formatted, and the wording is a bit off. You can use the preview functionality before posting to make sure you get the look you wanted.

[peace-maker]

@Arusekk your request warrants its own PR as it is a pretty big refactor of this commandline tool. Instead of using the stdlib errno module, search through the constants module and filter out the error constants. Where would you get the strerror descriptions? I don't feel like this is an easy adaption to this one and would rather open a new issue to add a context argument to the errno tool.