Extended attribute support: get/set/list/removexattr

Plumbing for in-memory extended attributes exists internally, but the get/set/listxattr syscalls are not implemented.

Some extended attributes will need to be propagated to the remote filesystem. This will require extending 9p protocol, but should be straightforward.

We may want to enforce a whitelist of allowed attributes for remote filesystems, in order to prevent apps from setting potentially dangerous attributes, or reading attribute values that they should not be able to see.