sd-dhcp6-client: modernize dhcp6_option_parse()

sd-dhcp6-client: modernize dhcp6_option_parse()

- merge dhcp6_option_parse() with option_parse_hdr().
- do not assign/update any values on error.
- use assert() instead of assert_return(), as the assertions cannot
  be triggered by a library user.

(cherry picked from commit b89a3758e92894162e3c2dcb594a55acff3274d5)