Eventscripts: Make 40.fs_use use less processes and arguably clearer.

Eventscripts: Make 40.fs_use use less processes and arguably clearer.

* $fs can be parsed using shell prefix and suffix removal.

* df output can be parsed with a single call to sed.

  Failure is indicated by empty output from sed, so we check for that
  as the error condition, changing the associated message
  appropriately.

Signed-off-by: Martin Schwenke <martin@meltin.net>