Darkcoin: new package (v0.9.13.15)

Darkcoin is a privacy-centered crypto-currency.
Litecoin: Update to 0.9.3-preview5

Now Litecoin expression lives in a single file, litecoin.nix
nixos/peerflix: fix enable option description
nixos: add chronos service
tree: don't rely on $CC being set in the stdenv build environment

Fixes https://github.com/NixOS/nixpkgs/issues/5214.
Update dd-agent to 4.4.0
liboping: update 1.6.2 -> 1.8.0

According to NEWS, the changes are in the command line tools, not the
library, so I think dependees (only collectd) will work fine.

Also update homepage: http://verplant.org/liboping/ => http://noping.cc/
rustSnapshot: upgraded to last master
claws-mail: update from 3.10.1 to 3.11.1
netcdf: enabled HDF5, DAP, UDUNITS, and shared library
cran-packages: restructured
cran-packages: renamed the files from camelCase to kebab-case
cran-packages: sets hydraPlatforms of rpud and CARramps to none since those packages require non-free NVidia drivers
cran-packages: makes packages overridable
netcdf: removed unused configure flag
cran-packages: uses common variable names, doCheck and installFlags
cran-packages: marks broken packages `broken` instead of removeing them
cran-packages: updated sources
cran-packages: combined packages definition files
cran-packages: combined packages definition files further