socket: Tag all socket option comments consistently

[imp]

Summary

For all the socket options that have a fixed type, add the type of the
socket option arg to the comment. Most of them already had this, but a
few did not. The ones that don't have a tag use a variable length data
structure of some kind, and are beyond the scope of this commit.

Sponsored by: Netflix

[tuexen]

What about the IPPROTO_SCTP and IPPROTO_UDPLITE-level socket options?

[pouria]

sys/netinet6/in6.h

433

Why not fixing it?

[peter.lei_ieee.org]

sys/netinet/tcp.h

226

This could be struct tcp_fastopen: min size is an int but a full struct may be set. A get returns an int -- so, not sure if that falls into "variable length"

229

This can be struct tcp_function_set which is a fixed size struct.

230

Same here: struct tcp_function_set is fixed size.

[bms]

a number of these are things I introduced, no objection