Making the gpr_tmpfile's win32 version a bit more Windows-y.

build.json

src/core/support/env_win32.c

src/core/support/file_win32.c

src/core/support/string_win32.c

src/core/support/string_win32.h

vsprojects/vs2013/gpr.vcxproj

vsprojects/vs2013/gpr.vcxproj.filters