System.java (static): Set http.agent system property when not yet set.

       * java/lang/System.java (static): Set http.agent system property when
       not yet set.
       * gnu/java/net/protocol/http/Connection.java (static): Get httpAgent
       from system property inside AccessController.doPrivileged() call.
       (proxyPort): Made package private.
       (proxyInUse): Likewise.
       (proxyHost): Likewise.
       (userAgent): Likewise.

From-SVN: r85078