MFC Loader Fixes 2017q4p1: r324321,r324359,r324360,r324388,r324449,r324450,
r324451,r324452
r324321: This README file was quite relevant for FreeBSD 3 or 4. No more.
r324359: Add efi_devpath_is_prefix
r324360: Encapsulate ZFS preferences into efi_zfs_is_preferred
r324388: Unbreak building efiboot with MK_ZFS == no
r324449: Prefer ${LIBSTAND} to -lstand
r324450: Define SASRC and use it
r324451: Add missing -I${SASRC} in a couple of places so that stand.h
doesn't accidentally come in via host pollution.
r324452: Define LIBSA* and use them instead of overloaded LIBSTAND