ipn: allow FQDN in exit node selection

To match the format of exit node suggestions and ensure that the result
is not ambiguous, relax exit node CLI selection to permit using a FQDN
including the trailing dot.

Updates #12618

Change-Id: I04b9b36d2743154aa42f2789149b2733f8555d3f
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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