When reentering kdb, typically due to a bug causing trap or assert in

the code executed in the context of debugger, do not be ashamed to
inform loudly about the re-entry.  Also, print the backtrace before
obliterating current stack with longjmp, allowing the operator to see
a place which caused the bug.

The change should make it less mysterious debugging the ddb itself.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/kern/subr_kdb.c

@@ -503,6 +503,8 @@ kdb_reenter(void)
 	if (!kdb_active || kdb_jmpbufp == NULL)
 		return;
 
+	printf("KDB: reentering\n");
+	kdb_backtrace();
 	longjmp(kdb_jmpbufp, 1);
 	/* NOTREACHED */
 }