gh-94518: Rename group* to extra_group* to avoid confusion

[arhadthedev]

This PR addresses #94687 (comment):

Todo: replace groups_size/groups parameter names with extra_group_size/extra_groups across the whole file to avoid confusion of passers-by. It's necessary to mitigate a misleading name of setgroups that supposes replacement of not supplimentary group affinities, but all ones. The mislead results in seeing the omission of setgroups call for groups_size == 0 as a bug (like we should reset the inherited affinities but do nothing instead).

I'll do it in a separate PR because both this PR and gh-94519 are already big enough to clobber them with extra details.

Edit: this PR also unifies terminology by further renaming groups_list parameter and num_groups variable into extra_groups_packed and extra_group_size.

cc @gpshead

• Issue: Port 23-argument _posixsubprocess.fork_exec to Argument Clinic #94518

[arhadthedev]

Converted to draft because num_groups and groups_list seem to require renaming too.

[arhadthedev]

Converted to draft again to rerun tests because spurious PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'D:\\a\\cpython\\cpython\\build\\test_python_6372�\\test_python_worker_6400�'struck again on Windows x64.

[arhadthedev]

Removed spuriously added technical _packed suffix from a Python signature of fork_exec. It's relevant inside C code only (to distinguish PyObject and gid_t * variants of extra_groups).

However, a new extra_groups argument name retains because fork_exec arguments are positional-only.