RFR [9] 8044773: Refactor jdk.net API so that it can be moved out of the base module

[Chris Hegarty]

One of the remaining open issues in JEP 200 [1] is that the base module
exports the jdk.net package, thereby violating Principle 4 of JEP 200:
a Java SE module must not export any non-SE API packages without
qualification.

http://cr.openjdk.java.net/~chegar/8044773/
https://bugs.openjdk.java.net/browse/JDK-8044773

Summary:

 - The jdk.net package has been moved to the jdk.net module. The native
 code and implementation that provides support for extended socket
 options, currently just SO_FLOW_SLA, has been moved too. A private
 interface between the base module and the jdk.net module now supports
 registration and operation of extended options.

 - The native code has been simplified and cleaned up.

 - Test coverage has been updated to exercise socket options, both with
 the jdk.net module present, and absent. The use of the
 jdk.launcher.limitmods system property is temporary until jtreg with
 support for -limitmods is more widely available.

 - Thanks to Erik for helping with build support that now cooperates
 with the intention of the GenModuleInfoSource build tool, to support
 unqualified export additions from non-open code.

-Chris.

[1] https://bugs.openjdk.java.net/browse/JDK-8051618