Perform asynchronous connection if possible

When using native GnuTLS, we can now connect asynchronously, without
blocking Emacs if the remote server is slow to accept the connection
(or just times out).

Such connections are now identical to "network" (i.e. TLS-less)
connections, so I reverted jabber-starttls-connect to its previous
state: it is now used exclusively to connect using gnutls-cli external
processes.