MFC r332752: set kdb_why to "trap" when calling kdb_trap from trap_fatal

This will allow to hook a ddb script to "kdb.enter.trap" event.
Previously there was no specific name for this event, so it could only
be handled by either "kdb.enter.unknown" or "kdb.enter.default" hooks.
Both are very unspecific.

Having a specific event is useful because the fatal trap condition is
very similar to panic but it has an additional property that the current
stack frame is the frame where the trap occurred.  So, both a register
dump and a stack bottom dump have additional information that can help
analyze the problem.

I have added the event only on architectures that have trap_fatal()
function defined.  I haven't looked at other architectures.  Their
maintainers can add support for the event later.

Sample script:
kdb.enter.trap=bt; show reg; x/aS $rsp,20; x/agx $rsp,20

Sponsored by:	Panzura

1 files changed, 10 insertions, 2 deletions

diff --git a/sys/powerpc/powerpc/trap.c b/sys/powerpc/powerpc/trap.c
index e201352..d1dc325 100644
--- a/sys/powerpc/powerpc/trap.c
+++ b/sys/powerpc/powerpc/trap.c
@@ -389,11 +389,19 @@ trap(struct trapframe *frame)
 static void
 trap_fatal(struct trapframe *frame)
 {
+#ifdef KDB
+	bool handled;
+#endif
 
 	printtrap(frame->exc, frame, 1, (frame->srr1 & PSL_PR));
 #ifdef KDB
-	if (debugger_on_panic && kdb_trap(frame->exc, 0, frame))
-		return;
+	if (debugger_on_panic) {
+		kdb_why = KDB_WHY_TRAP;
+		handled = kdb_trap(frame->exc, 0, frame);
+		kdb_why = KDB_WHY_UNSET;
+		if (handled)
+			return;
+	}
 #endif
 	panic("%s trap", trapname(frame->exc));
 }