stans: Narrow the scope of includes and other flags

CFLAGS+= here affects *ALL* libsa files being built. However, this is
only needed for zfs.c, so define it only for this. Also, use the defines
from defs.mk. Move all the zfs.c include hacks together. Also, move the
-Wformat -Wall warnings that were added to CFLAGS+= to the individual
files instead for the same reason.

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