daemon: Invoke 'guix gc --list-busy' instead of 'list-runtime-roots'.

* nix/scripts/list-runtime-roots.in: Remove.
* guix/store/roots.scm (%proc-directory): New variable.
(proc-file-roots, proc-exe-roots, proc-cwd-roots)
(proc-fd-roots, proc-maps-roots, proc-environ-roots)
(referenced-files, canonicalize-store-item, busy-store-items): New
procedures, taken from 'list-runtime-roots.in'.
* nix/libstore/globals.hh (Settings)[guixProgram]: New field.
* nix/libstore/globals.cc (Settings::processEnvironment): Initialize
'guixProgram'.
* nix/libstore/gc.cc (addAdditionalRoots): Drop code related to
'NIX_ROOT_FINDER'.  Run "guix gc --list-busy".
* nix/local.mk (nodist_pkglibexec_SCRIPTS): Remove
'scripts/list-runtime-roots'.
* config-daemon.ac: Don't output nix/scripts/list-runtime-roots.
* build-aux/pre-inst-env.in: Don't set 'NIX_ROOT_FINDER'.
Set 'GUIX'.
* doc/guix.texi (Invoking guix gc): Document '--list-busy'.
* guix/scripts/gc.scm (show-help, %options): Add "--list-busy".
(guix-gc)[list-busy]: New procedure.
Handle the 'list-busy' action.