permission: Fix idempotency issues for DN parameters

[t-woerner]

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