proc_detach: use ptrace(PT_KILL) to kill the tracee

When MFC'ing commit dad11f990e to stable/12, the child would dump core
when dtrace exited.  It was getting SIGTRAP, even though proc_detach
sent a SIGKILL.  I could not find the reason for this difference in
behavior from main (and stable/13).  The present change, however, works
as expected, probably due the proc_wkilled special case in kern_ptrace.
It also seems like a more obvious approach.

While I'm here, fix two other issues in the previous code:

It would SIGKILL a tracee even in read-only mode.

It would SIGSTOP/SIGCONT the tracee if ptrace succeeded but errno happened
to be EBUSY for some other reason.

Reviewed by:	markj
MFC after:	1 week
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D41122

lib/libproc/proc_util.c

@@ -85,25 +85,21 @@ int
 proc_detach(struct proc_handle *phdl, int reason)
 {
 	int status;
+	int request;
 	pid_t pid;
 
 	if (phdl == NULL)
 		return (EINVAL);
 	if (reason == PRELEASE_HANG)
 		return (EINVAL);
-	if (reason == PRELEASE_KILL) {
-		kill(proc_getpid(phdl), SIGKILL);
-		goto free;
-	}
 	if ((phdl->flags & PATTACH_RDONLY) != 0)
 		goto free;
+	request = (reason == PRELEASE_KILL) ? PT_KILL : PT_DETACH;
 	pid = proc_getpid(phdl);
-	if (ptrace(PT_DETACH, pid, 0, 0) != 0 && errno == ESRCH)
-		goto free;
-	if (errno == EBUSY) {
+	if (ptrace(request, pid, 0, 0) != 0 && errno == EBUSY) {
 		kill(pid, SIGSTOP);
 		waitpid(pid, &status, WUNTRACED);
-		ptrace(PT_DETACH, pid, 0, 0);
+		ptrace(request, pid, 0, 0);
 		kill(pid, SIGCONT);
 	}
 free: