New function: config::inherit_from().

This is a complement to config::merge_with(), as it does a merge without overwriting.
This has a few uses in unit_types.cpp, and using this function makes the code a bit
easier to understand at a glance.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56094 75b3775d-3801-0410-9e12-e2e43121521d