It turns out the version of SetDllDirectoryA/W in the master branch was only a rough start by Xeno86 to a partial implementation. It supports setting the Dll Directory search path, but not resetting it by passing NULL as the parameter.
I have simplified SetDllDirectory*(...) down to SetEnvironmentVariable*("DLLPATH",...) and also added GetDllDirectory*. Now I'm testing new LoadLibrary* and LoadLibraryEx* code that will use DLLPATH to fully implement WinXP-Win7 alternate DLL search paths. The Win8 functions AddDllDirectory, RemoveDllDirectory, and SetDefaultDllDirectory are a bit tougher and will be added later.
For the time being, SetDllDirectoryA/W can be disabled in CORE.INI.