MFC r336919, r336924

r336919:
efirt: Add tunable to allow disabling EFI Runtime Services

Leading up to enabling EFIRT in GENERIC, allow runtime services to be
disabled with a new tunable: efi.rt_disabled. This makes it so that EFIRT
can be disabled easily in case we run into some buggy UEFI implementation
and fail to boot.

r336924:
Follow up to r336919 and r336921: s/efi.rt_disabled/efi.rt.disabled/

The latter matches the rest of the tree better [0]. The UPDATING entry has
been updated to reflect this, and the new tunable is now documented in
loader(8) [1].

Reported by: imp [0], Shawn Webb [1]