feat(libkernel): pread, pwrite, getdents and getdirentries on linux
- **pread/pwrite:** use the existing `NativePread`/`NativePwrite` and don't move the file offset. A null buffer with a non-zero size returns `EFAULT`, a negative offset `EINVAL`, and host failures go through `SceErrorFromErrno`.
- **getdirentries:**
- reads host `getdents64` records and converts them to the FreeBSD `struct dirent` layout the guest expects (`d_fileno` u32, `d_reclen` u16, `d_type` u8, `d_namlen` u8, NUL-terminated name, record padded to 4 bytes);
- `*basep` gets the directory position from before the read;
- when a record doesn't fit in the guest buffer, the descriptor is moved with `lseek` back to that entry's cookie, so the next call resumes there;
- if not even the first record fits, the call returns `EINVAL` without consuming anything, and a name longer than 255 bytes returns `ENAMETOOLONG`.
- **getdents** is `getdirentries` without `basep`.
- Adds the guest `EFAULT`, `EINVAL` and `ENAMETOOLONG` constants.
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