tcpopt: add symbol table for mptcp suboptions

tcpopt: add symbol table for mptcp suboptions

nft can be used t match on specific multipath tcp subtypes:

  tcp option mptcp subtype 0

However, depending on which subtype to match, users need to look up the
type/value to use in rfc8684. Add support for mnemonics and
"nft describe tcp option mptcp subtype" to get the subtype list.

Because the number of unique 'enum datatypes' is limited by ABI contraints
this adds a new mptcp suboption type as integer alias.

After this patch, nft supports all of the following:
 add element t s { mp-capable }
 add rule t c tcp option mptcp subtype mp-capable
 add rule t c tcp option mptcp subtype { mp-capable, mp-fail }

For the 3rd case, listing will break because unlike for named sets, nft
lacks the type information needed to pretty-print the integer values,
i.e. nft will print the 3rd rule as 'subtype { 0, 6 }'.

This is resolved in a followup patch.

Other problematic constructs are:
  set s1 {
    typeof tcp option mptcp subtype . ip saddr
    elements = { mp-fail . 1.2.3.4 }
  }

Followed by:
  tcp option mptcp subtype . ip saddr @s1

nft will print this as:
  tcp option mptcp unknown & 240) >> 4 . ip saddr @s1

All of these issues are not related to this patch, however, they also occur
with other bit-sized extheader fields.

Signed-off-by: Florian Westphal <fw@strlen.de>