According to man 3 SSL_shutdown / TLS, "If a unidirectional shutdown is
enough (the underlying connection shall be closed anyway), this first
call to SSL_shutdown() is sufficient."

Currently, an unidirectional shutdown is enough, since
gitno_ssl_teardown is called by gitno_close only. Do so to avoid further
errors (by misbehaving peers for example).

Fixes #1129.