net/netmon: use Monitor's tsIfName if set by SetTailscaleInterfaceName

Currently nobody calls SetTailscaleInterfaceName yet, so this is a
no-op. I checked oss, android, and the macOS/iOS client. Nobody calls
this, or ever did.

But I want to in the future.

Updates #15408
Updates #9040

Change-Id: I05dfabe505174f9067b929e91c6e0d8bc42628d7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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