cmd/tailscale: make file cp send files via tailscaled localapi

So Taildrop sends work even if the local tailscaled is running in
netstack mode, as it often is on Synology, etc.

Updates #2179 (which is primarily about receiving, but both important)

Change-Id: I9bd1afdc8d25717e0ab6802c7cf2f5e0bd89a3b2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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