Have [unit_type] variations (including [male]/[female]) inherit their parent's ID, even
if inherit=no.

If a variation is really supposed to have a blank ID, that can still be accomplished by
explicitly setting id="".


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56097 75b3775d-3801-0410-9e12-e2e43121521d