fix(ipv6-opts): RPL option drops reserved flag bits on rebuild and rejects RFC 6553 sub-TLVs #1199

Describe the bug
_read_opt_rpl and _make_opt_rpl in HOPOPT and IPv6_Opts have two gaps that predate #1198. Both apply to type 0x23 and type 0x63.

1. Reserved flag bits are dropped. Only O, R and F survive the read and the maker, so 3b00230401020000 rebuilds through from_data as 3b00230400020000. The rebuild is byte-exact only when the reserved bits are 0.
2. Sub-TLVs are rejected. hopopt.py:1006 and ipv6_opts.py:995 raise ProtocolError unless the option length is exactly 4. RFC 6553 §3 makes the length variable and says unrecognised sub-TLVs MUST be skipped. For example, 3b0123060102000000aa000000000000 gives ProtocolError: HOPOPT: [OptNo 35] invalid format.

Expected behavior
Reserved bits round-trip unchanged. An option longer than 4 octets parses, and its sub-TLV bytes are kept so that the rebuild is byte-exact.

Found by the #1198 cross-review. I re-ran both inputs on the #1198 branch.