Refactor the implementation of [base_unit].

The previous version could fail if a unit type had multiple base units, if it
was defined before those unit types, and if those unit types themselves had
base units. (The config merge would lose a base unit's base unit.)


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56092 75b3775d-3801-0410-9e12-e2e43121521d