ipn/ipnlocal: pull out dns.Config construction to its own func

In prep for other bug fixes & tests. It's hard to test when it was
intermingled into LocalBackend.authReconfig.

Now it's a pure function.

And rename variable 'uc' (user config?) to the since idiomatic
'prefs'.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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