Remove psutil dependency on Windows.

We install from source on Windows as the main supported distribution mechanism. psutil is an optional dependency and is the only module we currently depend on that needs to build Windows native code, which pulls in a big toolchain (MSVS, mingw or similar).
Fall back from os.getppid() to os.getpid() on Windows
Fix logic error in dependency exclusion clause
Fix shebang lines for non-dx executables in Debian package.

This is done in essentially the same way as for tarball releases. To
support this the parts of package.sh that used to do this are factored
out into a separate script.
Changelog and dxR version update for v0.56.1