Return the data left behind. Return the data left behind.

Return the data left behind. Return the data left behind.

    * glib.h:
    * garray.c: (g_array_free), (g_ptr_array_free),
    (g_byte_array_free): Return the data left behind.
    * gstring.c: (g_string_free): Return the data left behind.

    Changed the free calls that leave data behind so they
    return a pointer to the left-behind data, NULL if told not
    to leave anything behind. This makes these calls easier
    to use correctly, without any incompatible API change for
    callers that don't know about the return value. Of course,
    it would be even clearer if the free calls weren't dual-purpose
    in the first place.