@PGS/20120615

Sometimes it is helpful to run more than one servald instance on a given machine
for debugging purposes.  To make this easier, there is a dummy interface driver
that servald knows about.  To use it:

1. create an empty file, e.g., dummynet0, somewhere convenient

2. For each servald instance you wish to use it, set the interface specification to include the dummynet file. Use a specification like "+>pathtodummynetfile", where pathtodummynetfile is the relative path from the instance path of that servald instance to the dummynet file.  For example, you might run:
   % servald config set interfaces "+eth0,+>../dummynet0"

3. Run each servald instance.  They should now use the dummy network.

NOTE: Because dummynets are files, not sockets, poll/select does not work on them.  As a result the main overlay loop has slightly different behaviour and timing characteristics when a dummynet is in use.

TODO: Convert dummynet interface to use a unixdomain socket, and a simple dummynet server that reflects packets among the clients connected, so that all socket semantics (including use of poll/select) are preserved.