Uniform dhcpv4 and dhcpv6 Parse methods

[insomniacslk]

The dhcpv4 and dhcpv6 packages use two different conventions for option parsing. In dhcpv4 the option is parsed along with option code and length, in dhcpv6 these are handled earlier, and the Parse* methods only handle the data, assuming that the option code is the same as the used option.

Let's uniform the two interfaces for consistency.

[pmazzini]

Do you have any preference of one over the other?

[insomniacslk]

I prefer the dhcpv6 convention but happy to discuss

[pmazzini]

Leaving this open since we now need to simplify the dhcpv6 parsing using github.com/u-root/u-root/pkg/uio to match the improvements recently added to dhcpv4.