fusefs: various cleanups

• Eliminate fuse_access_param. Whatever it was supposed to do, it seems like it was never complete. The only real function it ever seems to have had was a minor performance optimization, which I've already eliminated.
• Make extended attribute operations obey the allow_other mount option.
• Allow unprivileged access to the SYSTEM extattr namespace when -o default_permissions is not in use.
• Disallow setextattr and deleteextattr on read-only mounts.
• Add tests for a few more error cases.

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