Switch network module to read /proc/net/dev

This fixes issue #610 by reading bandwidth usage per-interface from
/proc/net/dev instead of globally via /proc/net/netstat. It supports the
same matching logic as elsewhere, so setting interface to '*' should
display the same sum-total bandwidth usage as the previous
implementation.

include/modules/network.hpp

@@ -43,6 +43,7 @@ class Network : public ALabel {
   const std::string getNetworkState() const;
   void              clearIface();
   bool              wildcardMatch(const std::string& pattern, const std::string& text) const;
+  std::optional<std::pair<unsigned long long, unsigned long long>> readBandwidthUsage();
 
   int                ifid_;
   sa_family_t        family_;