The recently-added GIO tools is intended to be built on all platforms, so
adjust the code a bit to enable this:
-Use gssize instead of ssize_t, as ssize_t is not supported by all
compilers.
-Include io.h on Windows, and define STDIN_FILENO and STDOUT_FILENO if
necessary on Windows.
https://bugzilla.gnome.org/show_bug.cgi?id=768357
This command collects the various commandline utilities that
are currently shipped in gvfs, and unifies them under a single,
command-style binary.
The tools just use GIO APIs, so it makes sense for them to live here.
