ndisc-option: introduce generic NDisc option parser

ndisc-option: introduce generic NDisc option parser

It is not used in this commit, but will be used for parsing NDisc
options in Router Advertisement message and friends.

The parser does mostly equivalent to what currently we do in
sd-ndisc-router.c. Several notable differences are:
- also perse source and target link-layer address,
- refuse multiple captive portals,
- check if the captive portal is in safe characters, as previously we
  checked that in networkd-ndisc.c,
- dedup prefixes, routes, and pref64,
- limit the total number of options, for safety.