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
Minimize race conditions in t_iprop.py

Augment kpropd's debugging output so we can see the starting and
ending serial numbers of a sync, and check them in t_iprop.py's
wait_for_prop to make sure we're observing the correct sync each time.
Try to make sure we only send SIGUSR1 when kpropd is actually
sleeping, by waiting for it to print its pre-sleep message to stderr
after a full dump and after each sync.  This isn't perfect, as there
is a short window between printing to stderr and actually sleeping,
but we can't do any better until kpropd has a more reliable mechanism
for triggering immediate updates.