Update kadmin set_string document

Added 'otp' as an acceptable attribute name.  Added examples.
Also, x-ref kadmin document from otp.rst.
Avoid duplicate "/etc/krb5.conf" in profile path

If configure gets run with --sysconfdir=/etc, "/etc/krb5.conf" shows
up twice in the profile path, which causes its contents to be read
twice.  This can cause some confusing and possibly problematic
behavior.

Add some logic to configure.in to avoid adding the duplicate entry for
"/etc/krb5.conf".

Reported independently by Denis Vlasenko and Fredrik Tolf.

ticket: 3277
tags: pullup
target_version: 1.12.2