permission: Fix idempotency issues for DN parameters

The parameters

- subtree (ipapermlocation)
- target (ipapermtarget)
- targetto (ipapermtargetto)
- targetfrom (ipapermtargetfrom)

have not been idempotent as the result returned from permission_show was
a DN and not a string.

The find_permission function has been exetended to convert the values
for these parameters to strings.

Fixes: #1257