wgengine/magicsock: refactor maybeRebindOnError

Remove the platform specificity, it is unnecessary complexity.
Deduplicate repeated code as a result of reduced complexity.
Split out error identification code.
Update call-sites and tests.

Updates #14551
Updates tailscale/corp#25648

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/magicsock_plan9.go

@@ -5,8 +5,8 @@
 
 package magicsock
 
-// maybeRebindOnError performs a rebind and restun if the error is defined and
-// any conditionals are met.
-func (c *Conn) maybeRebindOnError(os string, err error) bool {
-	return false
+// shouldRebind returns if the error is one that is known to be healed by a
+// rebind, and if so also returns a resason string for the rebind.
+func shouldRebind(err error) (ok bool, reason string) {
+	return false, ""
 }