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