Add lock annotations to libcuse.

- Add annotations to the lock/unlock function to indicate that the
  function is allowed to lock and unlock the underlying pthread mutex.

- Add __guarded_by() annotations to the global variables.

Approved by:	hselasky@


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270950 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f