Add :shutdown() method for ending the TLS session before :close()

[mwild1]

This allows applications to separate ending the TLS session from closing the underlying socket. In particular, it enables us to avoid needing to set the socket to blocking mode during close().

We need this to avoid a problem that can cause Prosody servers to block during connection close on some systems. More details in this prosody-dev mailing list post.

[alerque]

Is there any hope of getting test coverage on this?

[brunoos]

Sorry, I'm very busy these past days...

In general the patch seem to work, but I have some comments:

• In "case 0" of shutdown, it could return a string "inprogress" as error, instead of return nothing
• The connection state should be mark as closed in the case 0 also, since shutdown close the session immediately
• Shutdown state should use 3 state values instead of 0/1 and stop using the close state to control it

It would be good to look for if there is a way to recover the shutdown state directly from openssl, so as not to replicate the state in luasec