netns,wgengine: add OpenBSD support to netns via an rtable

When an exit node has been set and a new default route is added,
create a new rtable in the default rdomain and add the current
default route via its physical interface.  When control() is
requesting a connection not go through the exit-node default route,
we can use the SO_RTABLE socket option to force it through the new
rtable we created.

Updates #17321

Signed-off-by: joshua stein <jcs@jcs.org>

net/netmon/interfaces_bsd.go

@@ -4,7 +4,7 @@
 // Common code for FreeBSD and Darwin. This might also work on other
 // BSD systems (e.g. OpenBSD) but has not been tested.
 
-//go:build darwin || freebsd
+//go:build darwin || freebsd || openbsd
 
 package netmon