The "Skipped X from target list as found
unresponsive permanently" message was logged on
every `(*Cache).Check()` call for hosts with
permanent errors, resulting in thousands of
duplicate log entries in verbose mode.
Wrap the log statement in `sync.Once` to match the
behavior already used for non-permanent error
logging.
Signed-off-by: Dwi Siswanto <git@dw1.io>
127e03f572