Some cleanup

 I finally got around to rebuilding balance mod with clang 3.4 and
 strict warning options, and quashed a few warnings.

 With -pedantic, clang complains about lines 474 and 482 in
 src/uqm/supermelee/pickmele.c - where we cast
 calculate_crew_percentage to double (because we're using it for
 floating point math). This is actually not a problem, although the
 int literals are now float literals just to make what we're doing
 clear.