Baseline server support

Allows calling SSL_accept() instead of SSL_connect() when creating an
SslStream.
ssl: allow setting cipher list
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 `//`.