MFC: r287183, r287264, r287265

Export kern.features.invariants when kernel is compiled with invariants.

sys/kern/init_main.c

@@ -115,6 +115,10 @@ int	bootverbose = BOOTVERBOSE;
 SYSCTL_INT(_debug, OID_AUTO, bootverbose, CTLFLAG_RW, &bootverbose, 0,
 	"Control the output of verbose kernel messages");
 
+#ifdef INVARIANTS
+FEATURE(invariants, "Kernel compiled with INVARIANTS, may affect performance");
+#endif
+
 /*
  * This ensures that there is at least one entry so that the sysinit_set
  * symbol is not undefined.  A sybsystem ID of SI_SUB_DUMMY is never