Review Request JDK-8240975: Extend NativeLibraries to support explicit unloading

[Mandy Chung]

Webrev:
http://cr.openjdk.java.net/~mchung/jdk15/webrevs/8240975/webrev.00/

This is a follow-up task for Panama to allow explicit unloading of 
native library after JDK-8228336.  `NativeLibraries` associated with a 
class loader has the capability to auto unload native libraries when the 
class loader is reclaimed.  This adds a new `NativeLibraries::unload` 
method that provides the ability to unload a native library on request.  
This only allows  `NativeLibraries` of no relationship with a class 
loader where auto unloading is disabled.

Thanks
Mandy