SSL.ClientConnection: Improved SNI API.

Made SNI handling an explicit argument to create(), to allow for using
the same SSL.Context for client connections to multiple servers.
SSL.Connection: Fixed documentation typo.
SSL.sslfile: Changed client/server selection API.

Client and server operation is now selected by calling either
connect() (client-side) or accept() (server-side) after creating
the SSL.sslfile object.

Blocking handshaking mode is selected by calling set_blocking()
before either of the above.
SSL.sslfile: Support blocking accept().

accept() in blocking mode now waits for the handshake to complete.

Also adds some robustness at destruct time against not having any
connection.
Lower case exensions to match JPG images from cameras.