Add a simple EFI stub loader. This is a quick and dirty of boot1.chrp from
the PowerPC port with all the Open Firmware bits removed and replaced by
their EFI counterparts. On the whole, I think I prefer Open Firmware.

This code is supposed to be an immutable shim that sits on the EFI system
partition, loads /boot/loader.efi from UFS and tells the real loader what
disk/partition to look at. It finds the UFS root partition by the somewhat
braindead approach of picking the first UFS partition it can find. Better
approaches are called for, but this works for now. This shim loader will
also be useful for secure boot in the future, which will require some
rearchitecture.
Fix a panic in zfs_rename().
this is due to a wrong dereference of a vnode when it's not locked and
can be (potentially) recycled. 'sdvp' cannot be locked on zfs_rename()
entry point because the VFS can't be sure that this scenario is
LOR-free (it might violate the parent->child lock acquisition rule).
Dereference 'tdvp' instead, which is already locked on entry, and access
'sdvp' fields only when it's safe, i.e. under ZFS_ENTER scope.

While at it, remove the usage of VOP_REALVP, as long as this is a NOP
on FreeBSD.

Discussed with:	avg
Reviewed by:	pjd