Win32: Disable warnings about insecure functions

Set the _CRT_SECURE_NO_WARNINGS macro so that VC++ doesn't complain
about commonly-used functions like open and vsnprintf that it considers
"unsafe". The alternatives it suggests are not available on Unix-based
platforms, and they're used extensively throughout the codebase.
Win32: Include windows.h (indirectly) in all files
Win32: Add DFErrorSetWin32

This should ultimately go in a platform-specific file, but has been put
in DFError.c for now as an easy temporary measure.
DFHashTableAddInt: Don't turn a value (it's void)