conf: fix sourcedir reloading to not multiply sources

conf: fix sourcedir reloading to not multiply sources

The sourcedir reload triggered by the chronyc "reload sources"
command incorrectly assumed that NSR_AddSourceByName() can return
only the NSR_Success status when a source is added. It ignored the
NSR_UnresolvedName status returned for a source whose name needs to
be resolved after the call (i.e. not specified with an IP address)
and added the source again, effectively multiplying it if the name
can be resolved to a different IP address.

Fix the code to check for the NSR_UnresolvedName status to correctly
determine whether the source was already added before and should not be
added again.

Reported-by: MichaelR <MichaelR42@runbox.com>
Fixes: 916ed70c4a81 ("conf: save source status in sourcedir reload")