Rollup merge of #117957 - the8472:pidfd-wait, r=Mark-Simulacrum
if available use a Child's pidfd for kill/wait
This should get us closer to stabilization of pidfds since they now do something useful. And they're `CLOEXEC` now.
```
$ strace -ffe clone,sendmsg,recvmsg,execve,kill,pidfd_open,pidfd_send_signal,waitpid,waitid ./x test std --no-doc -- pidfd
[...]
running 1 tests
strace: Process 816007 attached
[pid 816007] pidfd_open(816006, 0) = 3
[pid 816007] clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f0c6b787990) = 816008
strace: Process 816008 attached
[pid 816007] recvmsg(3, <unfinished ...>
[pid 816008] pidfd_open(816008, 0) = 3
[pid 816008] sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="", iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[3]}], msg_controllen=24, msg_flags=0}, 0) = 0
[pid 816007] <... recvmsg resumed>{msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="", iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[4]}], msg_controllen=24, msg_flags=MSG_CMSG_CLOEXEC}, MSG_CMSG_CLOEXEC) = 0
[pid 816008] execve("/usr/bin/false", ["false"], 0x7ffcf2100048 /* 105 vars */) = 0
[pid 816007] waitid(P_PIDFD, 4, <unfinished ...>
[pid 816008] +++ exited with 1 +++
[pid 816007] <... waitid resumed>{si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=816008, si_uid=1001, si_status=1, si_utime=0, si_stime=0}, WEXITED, NULL) = 0
[pid 816007] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=816008, si_uid=1001, si_status=1, si_utime=0, si_stime=0} ---
[pid 816007] clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLDstrace: Process 816009 attached
, child_tidptr=0x7f0c6b787990) = 816009
[pid 816007] recvmsg(3, <unfinished ...>
[pid 816009] pidfd_open(816009, 0) = 3
[pid 816009] sendmsg(5, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="", iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[3]}], msg_controllen=24, msg_flags=0}, 0) = 0
[pid 816007] <... recvmsg resumed>{msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="", iov_len=0}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[5]}], msg_controllen=24, msg_flags=MSG_CMSG_CLOEXEC}, MSG_CMSG_CLOEXEC) = 0
[pid 816009] execve("/usr/bin/sleep", ["sleep", "1000"], 0x7ffcf2100048 /* 105 vars */) = 0
[pid 816007] waitid(P_PIDFD, 5, {}, WNOHANG|WEXITED, NULL) = 0
[pid 816007] pidfd_send_signal(5, SIGKILL, NULL, 0) = 0
[pid 816007] waitid(P_PIDFD, 5, <unfinished ...>
[pid 816009] +++ killed by SIGKILL +++
[pid 816007] <... waitid resumed>{si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=816009, si_uid=1001, si_status=SIGKILL, si_utime=0, si_stime=0}, WEXITED, NULL) = 0
[pid 816007] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=816009, si_uid=1001, si_status=SIGKILL, si_utime=0, si_stime=0} ---
[pid 816007] +++ exited with 0 +++
```
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