Never delete Subnets when StrictSubnets is set

Never delete Subnets when StrictSubnets is set

If a node is unreachable, and not connected to an edge anymore, it gets
deleted. When this happens its subnets are also removed, which should
not happen with StrictSubnets=yes.

Solution:
- do not remove subnets in src/net.c::purge(), we know that all subnets
  in the list came from our hosts files.
  I think here you got the check wrong by looking at the tunnelserver
  code below it - with strictsubnets we still inform others but do not
  remove the subnet from our data.
- do not remove nodes in net.c::purge() that still have subnets
  attached.