Restore support for building Tor in non-Unicode Windows installations

When we merged WinCE support in 8d31141c, we reportedly broke Win98 (!?) by requiring the Unicode versions of functions that previously had used the ascii variants.

The real fix here is to use the generic version of each WinAPI function, plus appropriate macros to make it build either with the UNICODE preprocessor macro defined or undefined.

In other words, where previously we said in our Windows code

    void func(const char *filename) {
        HANDLE library = LoadLibrary("filename.dll");
        CreateFile(filename, ...)
    }

and now since 8d31141c we say

    void func(const char *filename) {
        HANDLE library = LoadLibraryW(L"filename.dll");
        WCHAR wfilename[MAX_PATH]= {0};
        mbstowcs(wfilename,filename,MAX_PATH);
        CreateFileW(wfilename, ...)
    }

we should instead say

    void func(const char *filename) {
        HANDLE library = LoadLibrary(TEXT("filename.dll"));
#ifdef UNICODE
        WCHAR tfilename[MAX_PATH]= {0};
        mbstowcs(tfilename,filename,MAX_PATH);
#else
        const char *tfilename = filename;
#endif
        CreateFile(tfilename, ...)
    }

Thanks to mingw-san for pointing this out on bug legacy/trac#1715 (moved).