nixos: add programs.wireshark option

To be able to use Wireshark as an ordinary user, the 'dumpcap' program
must be installed setuid root. This module module simplifies such a
configuration to simply:

  programs.wireshark.enable = true;

The setuid wrapper is available for users in the 'wireshark' group.

Changes v1 -> v2:
  - add "defaultText" to the programs.wireshark.package option (AFAIK,
    that prevents the manual from being needlessly rebuilt when the
    package changes)