wgengine: simplify, change some signatures

* make RouterGen return an error, not take both tunname and tundev
* also remove RouteGen taking a wireguard/device.Device; currently unused
* remove derp parameter (it'll work differently)
* unexport NewUserspaceRouter in per-OS impls, add documented wrapper

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