modules: HylaFAX server configuration

This commit adds the following
* the uucp user
* options for HylaFAX server to control startup and modems
* systemd services for HylaFAX server processes
  including faxgettys for modems
* systemd services to maintain the HylaFAX spool area,
  including cleanup with faxcron and faxqclean
* default configuration for all server processes
  for a minimal working configuration

Some notes:

* HylaFAX configuration cannot be initialized with faxsetup
  (as it would be common on other Linux distributions).
  The hylafaxplus package contains a template spool area.
* Modems are controlled by faxgetty.
  Send-only configuration (modems controlled by faxq)
  is not supported by this configuration setup.
* To enable the service, one or more modems must be defined with
  config.services.hylafax.modems .
* Sending mail *should* work:
  HylaFAX will use whatever is in
  config.services.mail.sendmailSetuidWrapper.program
  unless overridden with the sendmailPath option.
* The admin has to create a hosts.hfaxd file somewhere
  (e.g. in /etc) before enabling HylaFAX.
  This file controls access to the server (see hosts.hfaxd(5) ).
  Sadly, HylaFAX does not permit account-based access
  control as is accepts connections via TCP only.
* Active fax polling should work; I can't test it.
* Passive fax polling is not supported by HylaFAX.
* Pager transmissions (with sendpage) are disabled by default.
  I have never tested or used these.
* Incoming data/voice/"extern"al calls
  won't be handled by default.
  I have never tested or used these.

nixos/modules/module-list.nix

@@ -517,6 +517,7 @@
   ./services/networking/heyefi.nix
   ./services/networking/hostapd.nix
   ./services/networking/htpdate.nix
+  ./services/networking/hylafax/default.nix
   ./services/networking/i2pd.nix
   ./services/networking/i2p.nix
   ./services/networking/iodine.nix