Replace all instances of ssize_t with gssize

ssize_t is supported widely, but not universally, so use gssize instead.
Currently only one piece of code actually *needs* this change to be compilable
with MSVC, the rest are mostly in *nix parts of the code, but these are changed
too, for symmetry.

https://bugzilla.gnome.org/show_bug.cgi?id=788180

gio/tests/dbus-launch.c

@@ -38,7 +38,7 @@ write_all (const void *ptr,
 
   while (len > 0)
     {
-      ssize_t done = write (STDOUT_FILENO, p, len);
+      gssize done = write (STDOUT_FILENO, p, len);
       int errsv = errno;
 
       if (done == 0)