FONTCONFIG_FILE: remove setters to /etc/fonts/fonts.conf

Any reasonably new version of fontconfig does search that path by default,
and setting this globally causes problems, as 2.10 and 2.11 need
incompatible configs.

Tested: slim+xfce desktop, chrootenv-ed steam.
I have no idea why we were setting the global variable;
e.g., neither Fedora nor Ubuntu does that.
kde-workspace: Use automatic version (close #4383)

See https://github.com/NixOS/nixpkgs/pull/4265#discussion_r18430068

This should work but I didn't test it :grin:
rustcMaster: Update to more recent commit and remove patch

The patch to the tests is now included upstream. Also in this version,
an issue was fixed upstream where LTO would fail with permission denied
in NixOS.
XFCE battery plugin: new package

Closes #4386
nixos/lighttpd: add services.lighttpd.enableModules option

This option makes the coupling between lighttpd and its sub-services
more "loose".

While the option is a list, its purpose is to provide a "set" of needed
modules to load for lighttpd to function correctly with its config. The
NixOS lighttpd module ensures that lighttpd modules are loaded no more
than once (because lighttpd dislikes that), and in the correct order.

Also add an assertion that all modules listed in .enableModules are
valid.