next attempt: make projectiles SOLID_CORPSE so they reliably hit anything, but

next attempt: make projectiles SOLID_CORPSE so they reliably hit anything, but
cannot be hit by player/spectator movement.  SOLID_TRIGGER failed at this
because these move with a only-bmodels trace.  SOLID_BBOX failed because a
SOLID_NOT moving into a SOLID_BBOX causes a hit, and also blocks the
SOLID_BBOX. SOLID_CORPSE is chosen because players have a hit contents mask
that allows them to move into SOLID_CORPSE.

Due to an override hit contents mask, the SOLID_CORPSE projectiles still
detonate on impact to "other" corpses (including other projectiles, although
this is unlikely).

git-svn-id: svn://svn.icculus.org/nexuiz/trunk@7283 f962a42d-fe04-0410-a3ab-8c8b0445ebaa