netstat, portlist: update Windows implementation to disambiguate svchost processes

We change our invocations of GetExtendedTcpTable to request additional
information about the "module" responsible for the port. In addition to pid,
this output also includes sufficient metadata to enable Windows to resolve
process names and disambiguate svchost processes.

We store the OS-specific output in an OSMetadata field in netstat.Entry, which
portlist may then use as necessary to actually resolve the process/module name.

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

net/netstat/netstat_noimpl.go

@@ -6,6 +6,10 @@
 
 package netstat
 
+// OSMetadata includes any additional OS-specific information that may be
+// obtained during the retrieval of a given Entry.
+type OSMetadata struct{}
+
 func get() (*Table, error) {
 	return nil, ErrNotImplemented
 }