Fix warnings glib

This patchset is fixing several compilation warnings in glib/. Note that:

• I closed the original werror MR and I will split it into smaller chunks, and do it one after one.
• I did set apart the patch about ghash.c because the change of the internal data-structure (gint -> gsize for size field) was breaking several tests. It will require a bit more work than I expected, but this is my next item in my TODO list.
• All these patches have already been reviewed, so it should be easy.