After r290196, the kernel won't wait for stuff like gmirror nodes

After r290196, the kernel won't wait for stuff like gmirror nodes
if they are not required for mounting rootfs.  However, it's possible
that some setups try to mount them in mountcritlocal (ie from fstab).

Export the list of current root mount holds using a new sysctl,
vfs.root_mount_hold, and make mountcritlocal retry if "mount -a" fails
and the list is not empty.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D3709