Handle zero foreground process groups on non-Linux targets

[jadenfix]

On macOS a newly opened pseudo-terminal can return zero from tcgetpgrp. The libc backend currently passes that to Pid::from_raw_unchecked, which requires a positive value. Apply the existing Linux zero-to-OPNOTSUPP handling on other libc targets too, and update the public documentation and Apple regression expectation.

Fixes #1678.

Validation on macOS:

• The existing pgrp_pseudoterminal test panics on the baseline; it passes after the fix.
• cargo test --features=all-apis: 215 unit/integration tests and 23 doctests passed, 3 tests intentionally ignored.
• Focused release process-group tests, cargo check --no-default-features --features=pty,termios, and cargo fmt --all -- --check passed.
• A separate public consumer checked a newly opened PTY (OPNOTSUPP), /dev/null (NOTTY), and an actual controlling terminal in a child process (the expected positive process group).
• Ordinary all-API documentation builds. An additional RUSTDOCFLAGS="-D warnings" check reports the same 13 existing errors on the clean baseline and candidate. Other operating systems and Rust 1.65 were not run locally.

Prepared with Codex assistance and independently reviewed by a separate Codex agent. No human review is claimed.

[xtqqczze]

No human review is claimed.

@jadenfix Could you please review your own contribution carefully before requesting human review?

[jadenfix]

Thanks for the reminder. I've personally reviewed this contribution.