Don't clear p_ptevents on normal SIGKILL delivery

[badger]

Summary

The ptrace() user has the option of discarding the signal. In such a
case, p_ptevents should not be modified. If the ptrace() user decides to
send a SIGKILL, ptevents will be cleared in ptracestop(). procfs events
do not have the capability to discard the signal, so continue to clear
the mask in that case.

Originally brought up in D9260.

Test Plan

It can be verified by configuring gdb to not pass SIGKILL that, without
this patch, sending a SIGKILL will result in lost LWP events. All
ptrace kyua tests still pass.

[jhb]

It would be nice to add a new test case for this if it isn't too much trouble.

sys/kern/kern_sig.c

2183

I would maybe keep the SIGKILL: prefix

[badger]

In D9939#205622, @jhb wrote:

It would be nice to add a new test case for this if it isn't too much trouble.

I could do that.

sys/kern/kern_sig.c

2183

I thought it was perhaps approaching the banal threshold due to its proximity to the sig == SIGKILL test, but it doesn't bother me to put it back.

[badger]

• Add kyua test
• restore SIGKILL: in comment