[libc] Add getpwent_r entrypoint (#226966)

Add the reentrant password database iteration entrypoint getpwent_r.

getpwent_r reads the next password database record from the stream into
the caller-supplied struct passwd and buffer, returning 0 on success,
ENOENT at end-of-file, or an error number (such as ERANGE) on failure.
When a record exceeds the caller buffer size, FlatFileDatabase::getnext
rewinds the stream to the beginning of that record so that a subsequent
retry with a larger buffer reads the same entry.

* Add getpwent_r entrypoint and pwd::read_next fixed-buffer overload
* Rewind stream on ERANGE in FlatFileDatabase::getnext(EntryType *,
span<char>)
* Define getpwent_r in include/pwd.yaml and Linux entrypoints.txt
* Add unit tests for getpwent_r

Assisted-by: Automated tooling, human reviewed.
GitOrigin-RevId: 844f9339a276cf18b7aac38c89f02a0d42e2339b