gio: Fix various implicit conversions from size_t to smaller types

Basically various trivial instances of the following MSVC compiler
warning:
```
../gio/gio-tool-set.c(50): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
```

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

gio/gdbusutils.c

@@ -113,7 +113,7 @@ is_valid_name (const gchar *start,
 gboolean
 g_dbus_is_name (const gchar *string)
 {
-  guint len;
+  size_t len;
   gboolean ret;
   const gchar *s;