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.