[libc] Add <unistd.h> functions for managing foreground process group IDs (#220121)

* Add POSIX functions `tcgetpgrp` and `tcsetpgrp` that get/set process
group IDs of foreground process groups associated with a terminal;
* Implement these functions as `ioctl` syscall wrappers and enable them
on Linux;
* Add unit test to verify the expected errno values for common error
cases (`EBADF` / `ENOTTY`),
but don't verify the "positive" cases, since it's challenging to do in a
unit test environment w/o using fork and
  creating pseudo-terminal fixtures.

Assisted by automated tooling, human-verified.

GitOrigin-RevId: 4bb9664e368b166fca6b9b6186f6d0a9f7cd3b91