A step towards removing more 'slave' terminology.

Overload the mechanisms for obtaining the jenkins-agent.jnlp (formerly slave-agent.jnlp) file for launching inbound TCP agents.
Serve the file at either of the two locations.
Change the UI (etc.) documentation to reference the correct term / file.

core/src/main/java/hudson/Functions.java

@@ -1874,7 +1874,7 @@ public class Functions {
     /**
      * Obtains the host name of the Hudson server that clients can use to talk back to.
      * <p>
-     * This is primarily used in {@code slave-agent.jnlp.jelly} to specify the destination
+     * This is primarily used in {@code jenkins-agent.jnlp.jelly} to specify the destination
      * that the agents talk to.
      */
     public String getServerName() {