1. the getpgrp function can help to pass the ltp/open_posix_teststuite/killpg related testcases
2. Nuttx do not support process group, so we use getpid to implement this
3. the implementation are referred to: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getpgrp.html
Signed-off-by: guoshichao <guoshichao@xiaomi.com>
