Ticket #47391 - deleting and adding userpassword fails to update the password

Bug description: Deleting and adding password operation causes
the crash since once an entry is written to and retrieved from
DB, it does not have unhashed#user#password in the entry any
more. The delete userpassword internally invokes delete unhashed
existed in the entry.

Fix description: Once the internal attribute value pair is
stored in an entry extension, the mod for the attribute is
changed to the "disabled" mode.  Also, this patch adds the
stricter check for the NULL reference.

https://fedorahosted.org/389/ticket/47391

Reviewed by Nathan (Thank you!!)
(cherry picked from commit 4305deeaa41ea8703ebb9ea6529790dd609b3060)
Ticket #47391 - deleting and adding userpassword fails to update the password (additional fix)

Bug description: ldapmodify with changetype "modify" is supposed
to skip checking unhashed password in acl_check_mods.  "delete"
and "replace" were being skipped, but not "add".

Fix description: "add" also skips to check unhashed password.

https://fedorahosted.org/389/ticket/47391

Reviewed by Rich (Thank you!!)
(cherry picked from commit 5337dcfa67827ac46df68a2f817eade638eb352d)