boinc service: use an FHS environment

While BOINC itself is open source, many of the project applications
it runs are not. Additionally, these project applications are
checksummed before they are run, so they can't be patched. This
means we can't make the project applications find required binaries/libraries
on a NixOS system. The solution is therefore to make said binaries
and libraries appear in the expected locations, by wrapping BOINC
in an FHS-compatible environment [1].

An `extraEnvPackages` is also added to allow more packages to
be added into this environment. The documentation for this option
describes some practical use cases for it.

[1] https://nixos.org/nixpkgs/manual/#sec-fhs-environments

Version 2: fixed Docbook list formatting, minor rewording

Version 3: rebase onto master (9c048f4f)

Version 4: fix usage of targetPkgs argument to buildFHSUserEnv