1. the killpg function can make all the
ltp/open_posix_testsuite/killpg testcaes passed
2. Nuttx do not support process group, so we use kill process instead
3. the implementation are referred to:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/killpg.html
Signed-off-by: guoshichao <guoshichao@xiaomi.com>
