wireshark: add patch to lookup "dumpcap" in PATH

What this allows us to do is define a "dumpcap" setuid wrapper in NixOS
and have wireshark use that instead of the non-setuid dumpcap binary
that it normally uses.

As far as I can tell, the code that is changed to do lookup in PATH is
only used by wireshark/tshark to find dumpcap. dumpcap, the thing that's
typically setuid, is not affected by this patch. wireshark and tshark
should *not* be installed setuid, so the fact that they now do lookup in
PATH is not a security concern.

With this commit, and the following config, only "root" and users in the
"wireshark" group will have access to capturing network traffic with
wireshark/dumpcap:

  environment.systemPackages = [ pkgs.wireshark ];
  security.setuidOwners = [
    { program = "dumpcap";
      owner = "root";
      group = "wireshark";
      setuid = true;
      setgid = false;
      permissions = "u+rx,g+x";
    }
  ];
  users.extraGroups.wireshark.gid = 500;

(This wouldn't have worked before, because then wireshark would not use
our setuid dumpcap binary.)