ssl verify mode flags: complete and convert to bitflags!

Add the server-only SslVerifyMode entries. Use bitflags! so we can or
them together easily.

I've updated the docs to mention server mode behavior, unfortunately
bitflags! doesn't seem to like `///` inside it, so I've replaced the
`///` with `//`.
It doesn't look like a correct flag. Haven't checked OpenSsl sources but expect it should be 4