stand: Rename LIBFDT to LIBSAFDT

Preemptively address a collision with LIBFDT (to be added in the future)
from src.libnames.mk, which gets included via bsd.progs.mk. No
functional change intended.

Reviewed by: imp
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D42486