Ticket #447 - Possible to add invalid attribute to nsslapd-allowed-to-delete-attrs

Bug description: If given value of nsslapd-allowed-to-delete-attrs are
all invalid attributes, e.g.,
  nsslapd-allowed-to-delete-attrs: invalid0 invalid1
they were logged as invalid, but accidentally set to nsslapd-allowed-
to-delete-attrs.

Fix description: This patch checks the validation result and if there
is no valid attributes given to nsslapd-allowed-to-delete-attrs, it
issues a message in the error log:
  nsslapd-allowed-to-delete-attrs: Given attributes are all invalid.
  No effects.
and it returns an error.  The modify operation fails with "DSA is
unwilling to perform".

https://fedorahosted.org/389/ticket/447

Reviewed by rmeggins@redhat.com (Thank you, Rich!)
(cherry picked from commit 31cd7a838aef30d80be6efe519cc2e821811c645)
(cherry picked from commit eab32225c129f6a5115bbd5ac2a3c2035f4393b2)
(cherry picked from commit c392aa891e67b8be189d3e354a179fc376998642)