ipn/ipnlocal: check operator user via osuser package

So non-local users (e.g. Kerberos on FreeIPA) on Linux can be looked
up. Our default binaries are built with pure Go os/user which only
supports the classic /etc/passwd and not any libc-hooked lookups.

Updates #12601

Change-Id: I9592db89e6ca58bf972f2dcee7a35fbf44608a4f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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