This makes it possible to list unix(4) protocols in -P, which was not
possible before. Update the manual page and provide an example.

In 'struct sock' make member proto a pointer into the protocols table
instead of a number. This removes namespace collision that mixed
IPPROTO_TCP and etc with SOCK_STREAM and etc. There were no known bugs
due to this collision, though.

Preserve all current UI bugs for possible combinations of protocol
selection options. Should be no UI changes outside the improved -P.