unassigned fibers should have DESI_TARGET = NO_TARGET

[sbailey]

desitarget.targetmask.desi_mask has a bit reserved for "NO_TARGET", meaning that we know that we're putting a fiber at a location that doesn't have a known target. Unassigned fibers from fiberassign should set that bit in DESI_TARGET to show that it was purposeful and not some bug that left DESI_TARGET=0 (e.g. see #221).

Whenever this happens for real data, what we really want to do is go back to the target catalogs and augment them with BAD_SKY, SUPP_SKY, or some other kind of backup target near those locations so that everything is assigned to something. But belt-and-suspenders: if we can't get a target, we should flag DESI_TARGET = NO_TARGET for the fake entries assigned to those fibers.