The --mesh-with flag now supports the specification of hostname tuples like
derp1a.tailscale.com/derp1a-vpc.tailscale.com, which instructs derp to mesh
with host 'derp1a.tailscale.com' but dial TCP connections to 'derp1a-vpc.tailscale.com'.
For backwards compatibility, --mesh-with still supports individual hostnames.
The logic which attempts to auto-discover '[host]-vpc.tailscale.com' dial hosts
has been removed.
Updates tailscale/corp#25653
Signed-off-by: Percy Wegmann <percy@tailscale.com>