wgengine/magicsock: probe but don't use path MTU from CLI ping

When sending a CLI ping with a specific size, continue to probe all possible UDP
paths to the peer until we find one with a large enough MTU to accommodate the
ping. Record any peer path MTU information we discover (but don't use it for
anything other than CLI pings).

Updates #311

Signed-off-by: Val <valerie@tailscale.com>
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