Rename unit_type::race() to unit_type::race_id().

This is for more consistent naming of functions (this function returns a string,
while unit::race() returns a pointer to unit_race).


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@55998 75b3775d-3801-0410-9e12-e2e43121521d