fix MSVC compilation warnings in apps

* pdfapp_save in pdfapp.c contains unreachable code (and could be simplified by inverting most of the checked conditions)
* RegSetValueExA expects a const BYTE pointer instead of a char pointer (warns about signedness mismatch)
* same for SendMessageA where -1 is signed even though WPARAM isn't

Also, there's no need to include mupdf.h, muxps.h, etc. at the top of applications which only use the fz_* API (fitz.h comes included with pdfapp.h).