Fix for issue #2302 - NativeLibraryLoader fails due to no write permissions

[terranprog]

This fixes an an issue I encountered while running a jmonkey application in Linux. #2302 - NativeLibraryLoader fails due to no write permissions.
The cause of the bug is a bit of an edge case, but it results in the native library not being extracted and the application not starting.

The main change I made is to extract the native library to a folder directly beneath the user temp folder, instead of using an intermediate tmp/jme subfolder. The reason for this is to avoid the scenario where the /tmp/jme folder is owned by one user and another user tries to write to it.
Note that this change is optional and I could modify the PR to not include it. We can still use /tmp/jme and fall back to calling setExtractionFolderToUserCache() if the /tmp/jme folder is not writable.

The other changes are to tighten the logic so the code will call setExtractionFolderToUserCache() instead of crashing.

• Check the result of the call to mkdir. If it fails, call call setExtractionFolderToUserCache()
• Check if the extraction folder is actually writable. If not, call setExtractionFolderToUserCache()
• Updated log message: since a previous change we no longer extract to the working directory.

Some general notes about this PR:
I tried to limit the scope to only fix the immediate issue.
There are other issues with NativeLibraryLoader that this PR does not address

• There are other places in NativeLibraryLoader where the result of mkdir is not checked.
• It should potentially use mkdirs() instead of mkdir()