Remove use of operator[] on a std::map, so no spurious elements get inserted. (fixes #464)

All uses of operator[] in guiConfigureWorld.cpp have been replaced
with .find() operations.

In some places, operator[] had been called with mod names of "Add-Ons"
or "N/A", which would insert a default-constructed ModSpec into the
list of add-on mods. In other places, the use of operator[] was safe
now, but it probably is safer for future changes to use .find()
everywhere.
Restore old acceleration behavior until something is actually agreed on
Restore old acceleration behavior until something is actually agreed on (part 2)
Some kind of middleway values for player acceleration for now
Merge pull request #465 from doserj/mod_selection_empty_modname_fix

Remove use of operator[] on a std::map, so no spurious elements get inse...
Merge branch 'master' of https://github.com/celeron55/minetest
Added lua control for the option of not staticizing an entity