GVariant: avoid byteswapping in some cases

GVariant: avoid byteswapping in some cases

Make g_variant_byteswap() merely return a new reference on the given
value in the event that we know that byteswapping will have no effect
(ie: types which have no alignment requirement).

This fixes a somewhat complicated interaction between GVariant,
GSettings and GVDB on big endian machines:  GSettings assumes that it
can unref values returned from GVDB without losing access to the
underlying data.  This only works if the underlying data is in the
mapped file -- not a freshly-allocated buffer that GVariant byteswapped
into.