Added nebu_absorb_shield shipstat

Added nebu_absorb_shield shipstat, as well as a prototype outfit, nebula_shielding0,
that uses it.  If a ship has nebu_absorb_shield, it will be applied to nebula damage
in the same way that dmg_absorb is applied to normal damage.  Removed nebula_dmg_shield,
nebula_dmg_armor, since they were presumably meant to do the same thing, but didn't
have any real code attached to them in the damage calcs.  The Thurion Integrity will
need to be updated, since it uses the old values, and since they didn't do anything,
I have no way of knowing what -80 and -97 were supposed to mean in that context.
I also added a nebu_absorb_armor shipstat as a placeholder, but it doesn't do anything
yet.
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/rkulhanek/naev