Forced SSL version selection added.

Both client's and server's instances can specify SSL version to use.
This option may be used as a enum config value for transport_ssl_t:
TLSv1, TLSv1_1, SSLv2, SSLv3, SSLv23 (default)
This option changes the behaviour of the handshake and the protocols
which can used for communication
With the default SSLv23 value server-side SSLv2 protocol is forbidden.

Generally the default SSLv23 value is enough. Client will send SSLv2
handshake and indicate the use of protocols SSLv3 and TLSv1. Server
accepts SSLv2, SSLv3, TLSv1 handshakes. Best choice when compatibility
is concern.

Configuration example:
	...
	transport_t ssl_transport = transport_ssl_t {
		timeout = 1s
		ssl_version = TLSv1_1
	}
	transport = ssl_transport
	...