wgengine/magicsock: close disco listeners on Conn.Close, fix Linux root TestNewConn

TestNewConn now passes as root on Linux. It wasn't closing the BPF
listeners and their goroutines.

The code is still a mess of two Close overlapping code paths, but that
can be refactored later. For now, make the two close paths more similar.

Updates #9945

Change-Id: I8a3cf5fb04d22ba29094243b8e645de293d9ed85
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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