SslRMIServerSocketFactory should accept an SSLContext as input

Currently the implementation of the standard SslRMIServerSocketFactory relies
on the default implementation of the SSLSocketFactory.getDefault() method.
This means that it must be initialized using the javax.net.ssl.keyStore* and
javax.net.ssl.trustStore* system properties. This is not always convenient
and makes the current implementation of the SslRMIServerSocketFactory almost
unusable because all the RMI code wanting to use SSL encryption and which runs
in the same VM must share the same key and trust materials.

Adding a new constructor that takes an SSLContext would allow customer code to
initialize the keystores and truststores independently for each RMI exported
object.