RFR: JDK-8034179: Clean up nio genConstants

[Erik Joelsson]

Here is a second patch for cleaning up gensrc tools source. This time 
it's nio related.

* Move genUnixConstants.c, genSolarisConstants.c and 
genSocketOptionRegistry.c to make/src/native/...
* Rename pregenerated versions of these generated files to 
*.java.template to avoid having to exclude them when compiling java code.
* Move the output of the executable binaries to 
$(JDK_OUTPUTDIR)/btnative away from the gensrc dir where we expect to 
find java sources.

Bug: https://bugs.openjdk.java.net/browse/JDK-8034179
Webrev: http://cr.openjdk.java.net/~erikj/8034179/webrev.jdk.01/

/Erik