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>