o add a mountroot event handler that fires when / is mounted; this information

was lost when root started being mounted by init
o remove SI_SUB_MOUNT_ROOT since it's no longer meaningful

MFC after:	2 weeks

sys/kern/vfs_mount.c

@@ -1488,6 +1488,8 @@ set_rootvnode(struct thread *td)
 	FILEDESC_SUNLOCK(p->p_fd);
 
 	VOP_UNLOCK(rootvnode, 0);
+
+	EVENTHANDLER_INVOKE(mountroot);
 }
 
 /*

sys/sys/eventhandler.h

@@ -171,6 +171,10 @@ EVENTHANDLER_DECLARE(vm_lowmem, vm_lowmem_handler_t);
 typedef void (*vfs_lowvnodes_handler_t)(void *, int);
 EVENTHANDLER_DECLARE(vfs_lowvnodes, vfs_lowvnodes_handler_t);
 
+/* Root mounted event */
+typedef void (*mountroot_handler_t)(void *);
+EVENTHANDLER_DECLARE(mountroot, mountroot_handler_t);
+
 /*
  * Process events
  * process_fork and exit handlers are called without Giant.

sys/sys/kernel.h

@@ -153,7 +153,6 @@ enum sysinit_sub_id {
 	SI_SUB_ROOT_CONF	= 0xb000000,	/* Find root devices */
 	SI_SUB_DUMP_CONF	= 0xb200000,	/* Find dump devices */
 	SI_SUB_RAID		= 0xb380000,	/* Configure GEOM classes */
-	SI_SUB_MOUNT_ROOT	= 0xb400000,	/* root mount*/
 	SI_SUB_SWAP		= 0xc000000,	/* swap */
 	SI_SUB_INTRINSIC_POST	= 0xd000000,	/* proc 0 cleanup*/
 	SI_SUB_SYSCALLS		= 0xd800000,	/* register system calls */