Adds `ZEROTIER_ONE_GATEWAY_MODE` variable. Supported values are
`inbound` (forward traffic from ZeroTier cloud to local interfaces),
`outbound` (forward traffic from local interfaces to ZeroTier cloud)
and `both` (bi-directional). Defaults to `inbound`.
Also checks for `net.ipv4.ip_forward=1`. If not enabled, falls back
to standard client mode.
Signed-off-by: Phill Kelley <34226495+Paraphraser@users.noreply.github.com>
