Don't intercept tactical and marine missiles targeting planes

Don't intercept tactical and marine missiles targeting planes

Missiles attacking ground targets (sectors, ships or land units) can
be intercepted, missiles targeting planes can't.  Except msl_launch()
checked the missile's capabilities instead of its target, and thus
intercepted missiles that *could* target ground even when they
targeted planes.

This broke the missile interception code's assumption that
interceptors aren't intercepted, and crashed the server through
infinite recursion: tactical abm #1 intercepts, tactical abm #2
intercepts #1, #1 intercepts #2, ...

Same bug in msl_hit() printed "incoming missile" for missiles that
could target ground but do target planes.

Broken in Empire 2.  Bug can't bite with the stock game's planes.