Vega Strike Code

Fix upgrade description generation to avoid bogus output with the
"recently" introduced splitting of units.csv, which makes a lot of
missing fields have default values. Detect no-ops (0 for additive) and
defaults, and skip those attributes in the listings.

Fix missiles not always impacting on target due to botched collision
checks. Fix missiles never damaging asteroids due to not checking for
subunits when applying damage. Add debugging output that is useful when
debugging those issues.

Fix querySphere to ignore meshless units that have subunits.
Fixes missiles exploding in your face if you fire them inside an
asteroid field.

Add some more debugging output, and other minor improvements

Optimize handling of subunit-less units by skipping some unneeded work

Fix segfault when building an iterator for a collection of killed units
The iterator was being registered too late (after calling erase) so
erase was performing unsafe list manipulations.

May fix #680 and related errors.

Telltale signs of this bug are crashes shortly after a bunch of subunits
get killed (UNIT HAS DIED in the last few lines of log output), which
happen more often than not when killing units with missiles.

Optimize condition for empty collections to avoid computing the size of
a linked list, which is O(N), whenever possible

Remove useless argument that's just the default value

Small profile-guided optimization: simplify GetWarpVelocity when not in warp

Small profile-guided optimization: Unit::nummesh() doesn't really need
to be virtual, and having it so is burning lots of cycles in hot
draw/physics loops, so until it becomes necessary, make it nonvirtual
and inline