Add support for alternative break-to-debugger to syscons(4).  While most
keyboards allow console break sequences (such as ctrl-alt-esc) to be
entered, alternative break can prove useful under virtualisation and
remove console systems where entering control sequences can be
unreliable.

MFC after:	3 weeks
Approved by:	re (xxx)

Index: dev/syscons/syscons.c
===================================================================
--- dev/syscons/syscons.c	(revision 225203)
+++ dev/syscons/syscons.c	(working copy)
@@ -3562,6 +3562,10 @@
 	    /* goto next_code */
 	} else {
 	    /* regular keys (maybe MKEY is set) */
+#if !defined(SC_DISABLE_KDBKEY) && defined(KDB)
+	    if (enable_kdbkey)
+		kdb_alt_break(c, &sc->sc_altbrk);
+#endif
 	    if (!(sc->flags & SC_SCRN_BLANKED))
 		return c;
 	}
Index: dev/syscons/syscons.h
===================================================================
--- dev/syscons/syscons.h	(revision 225203)
+++ dev/syscons/syscons.h	(working copy)
@@ -266,6 +266,9 @@
 	u_char		cursor_char;
 	u_char		mouse_char;
 
+#ifdef KDB
+	int		sc_altbrk;
+#endif
 } sc_softc_t;
 
 /* virtual screen */