Add a script to add git branches for each channel.
Add other source of channels, and distinguish between local and remote channels.
update-channel-branches.sh: Add verbosity to improve the user experience, and update NixOS documentation.
nixos-option: Print the outPath of derivation for option values.
nixos-option: Print derivation outPath within attribute sets and list, when the strict mode is used.
nixos-option: Improve error messages to avoid reporting internal location and traces.
Merge pull request #5362 from nbp/update-channels

Add a script to add git branches for each channel.

To create / update references to remote / local channels, you have to run `./maintainers/scripts/update-channel-branches.sh` while you are at the top-level of nixpkgs work directory.  To make this convenient for Nixpkgs / NixOS developer, one can run the following command:

```
$ git config --add alias.fetch-channels '!sh -c "$(git rev-parse --show-cdup)maintainers/scripts/update-channel-branches.sh"'
```

Which will register the alias fetch-channels such that the script can used from sub-directory of nixpkgs by running `git fetch-channels`.
Merge pull request #5405 from nbp/nixos-options-derivation

nixos-option: Print the outPath of derivation for option values.
qpdfview 0.4.12 → 0.4.13
lib odfgen 0.1.1 → 0.1.2
compiz: 0.9.11.2 → 0.9.12.0
eigen: 3.2.2 → 3.2.3
afflib: 3.7.5 → 3.7.6
SlimerJS: 0.9.3 → 0.9.4
sodium: 1.0.0 → 1.0.1
atanks: 6.0 → 6.1
libre: 0.4.10 → 0.4.11
Firejail: 0.9.16 → 0.9.18
LVM2: 2.02.111 → 2.02.114
xscreensaver: 5.30 → 5.32