In the gdbus port, the :options properties will be GVariant-valued
(and thus immutable), so having APIs that let you repeatedly modify
them would make things complicated. Since we actually only ever set
all the options at once, just change the APIs to do that, rather than
setting the options one-by-one.

Since nm-dispatcher already works in terms of GVariant, it makes
things simpler there if NMDhcp[46]Config can return its options as a
GVariant. And since we'll need it to be a GVariant internally later
anyway, just port everything to GVariant now, and convert it to a
GHashTable for dbus-glib only in get_property().