The trusted keys option parsing allows specifying the same option
multiple times. The last option value specified is used.

This is problematic because:

* No gain.
* This makes complicated to specify options that are dependent on other
  options.

This patch changes the behavior in a way that option can be specified
only once.
Reported-by: NJames Morris James Morris <jmorris@namei.org>
Reviewed-by: NMimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: NJarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Acked-by: NPeter Huewe <peterhuewe@gmx.de>