More robust native library discovery in Mac OS X

Motivation:

Some JDK versions of Mac OS X generates a JNI dynamic library with '.jnilib' extension rather than with '.dynlib' extension.  However, System.mapLibraryName() always returns 'lib<name>.dynlib'. As a result, NativeLibraryLoader fails to load the native library whose extension is .jnilib.

Modification:

Try to find both '.jnilib' and '.dynlib' resources on OS X.

Result:

Dynamic libraries are loaded correctly in Mac OS X, and thus we can continue the OpenSslEngine work.