Fix MS-Windows MinGW build

Fix MS-Windows MinGW build

Most of the fixes are portability related.  Some functions
were moved from unexw32.c to w32proc.c, because they are
needed even if unexec is not used.
After these fixes, Emacs builds without warnings (GCC 6.3.0),
dumps itself, but then crashes when it tries to compile the
first Lisp file.