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

(cherry picked from commit b247ff70e8f1b4bf184b9fc85d2908ec4db2d1ab)