Rewrite Fontconfig NixOS module

Details:
* The option `fonts.enableFontConfig` has (finally) been renamed
  `fonts.fontconfig.enable`.
* Configurations are loaded in this order: first the Fontconfig-upstream
  configuration is loaded, then the NixOS-specific font directories are
  set, the system-wide default configuration is loaded, and finally the
  user configuration is loaded (if enabled).
* The NixOS options `fonts.fontconfig.defaultFonts.monospace`,
  `fonts.fontconfig.defaultFonts.sansSerif` and
  `fonts.fontconfig.defaultFonts.serif` are added to allow setting the
  default system-wide font used for these generic faces. The defaults
  are the appropriate faces from the DejaVu collection because of their
  comprehensive Unicode coverage, clean rendering, and excellent
  legibility.
* The NixOS option `fonts.fontconfig.antialias` can be used to disable
  antialiasing (it is enabled by default).
* The options `fonts.fontconfig.subpixel.rgba` and
  `fonts.fontconfig.subpixel.lcdfilter` control the system-wide default
  settings for subpixel order and LCD filtering algorithm,
  respectively.
* `fonts.fontconfig.hinting.enable` can be used to disable TrueType font
  hinting (it is enabled by default).
  `fonts.fontconfig.hinting.autohint` controls the FreeType autohinter.
  `fonts.fontconfig.hinting.style` controls the hint style; it is "full"
  by default.
* User configurations can be disabled system-wide by setting
  `fonts.fontconfig.includeUserConf = false`. They are enabled by
  default so users can set Fontconfig options in the desktop environment
  of their choice.