add static const members ZERO, default_dirs, of map_location

Replace local constants appearing elsewhere with these.
The definitions are inlined.
inline the definition map_location::null_location

If we decided it was worth it to inline map_location ==, then
surely this is also worth it to inline null_location, as the
compiler may be able to reduce tests for == null location to
"x == ... && y == ...", without having to move to a different
compilation unit.
fixup travis (failure to find preferences is not reported as error)