Add SSL iostream stub

This commit adds SSL context to iostream_ctx and a new kind of error for
SSL connections.

The SSL context is created if the target URI has transport=ssl
parameter. If transport=plain or absent, it will be set to NULL and
plain streams will be created as before. For other values of the
transport parameter, an error is raised. Note, this is just a stub -
an attempt to create an SSL context always currently fails with
"SSL not available" error. It is supposed to be implemented in
EE build.

The new kind of error is currently only used for decoding errors from
MsgPack and never raised. It will be raised by the actual implementation
of SSL stream.