src/daemon/common.[ch]: Improve signature of value_to_rate().

src/daemon/common.[ch]: Improve signature of value_to_rate().

Passing in "value" as a derive_t and returning the rate as a value_t is
just inconvenient and wrong. Instead, pass "value" as a value_t and return
the rate in a gauge_t. While at it, move the state pointer to the end of
the argument list.