monads: Move '%store-monad' and related procedures where they belong.

This turns (guix monads) into a generic module for monads, and moves the
store monad and related monadic procedures in their corresponding
module.

* guix/monads.scm (store-return, store-bind, %store-monad, store-lift,
  text-file, interned-file, package-file, package->derivation,
  package->cross-derivation, origin->derivation, imported-modules,
  compiled, modules, built-derivations, run-with-store): Move to...
* guix/store.scm (store-return, store-bind, %store-monad, store-lift,
  text-file, interned-file): ... here.
  (%guile-for-build): New variable.
  (run-with-store): Moved from monads.scm.  Remove default value for
  #:guile-for-build.
* guix/packages.scm (default-guile): Export.
  (set-guile-for-build): New procedure.
  (package-file, package->derivation, package->cross-derivation,
  origin->derivation): Moved from monads.scm.
* guix/derivations.scm (%guile-for-build): Remove.
  (imported-modules): Rename to...
  (%imported-modules): ... this.
  (compiled-modules): Rename to...
  (%compiled-modules): ... this.
  (built-derivations, imported-modules, compiled-modules): New
  procedures.
* gnu/services/avahi.scm, gnu/services/base.scm, gnu/services/dbus.scm,
  gnu/services/dmd.scm, gnu/services/networking.scm,
  gnu/services/ssh.scm, gnu/services/xorg.scm, gnu/system/install.scm,
  gnu/system/linux-initrd.scm, gnu/system/shadow.scm, guix/download.scm,
  guix/gexp.scm, guix/git-download.scm, guix/profiles.scm,
  guix/svn-download.scm, tests/monads.scm: Adjust imports accordingly.
* guix/monad-repl.scm (default-guile-derivation): New procedure.
  (store-monad-language, run-in-store): Use it.
* build-aux/hydra/gnu-system.scm (qemu-jobs): Add explicit
  'set-guile-for-build' call.
* guix/scripts/archive.scm (derivation-from-expression): Likewise.
* guix/scripts/build.scm (options/resolve-packages): Likewise.
* guix/scripts/environment.scm (guix-environment): Likewise.
* guix/scripts/system.scm (guix-system): Likewise.
* doc/guix.texi (The Store Monad): Adjust module names accordingly.

Merge branch 'master' into core-updates

services: Use system groups where applicable.

* gnu/services/avahi.scm (avahi-service): Add 'system?' field to
  'user-group' form.
* gnu/services/base.scm (guix-service): Likewise.
* gnu/services/dbus.scm (dbus-service): Likewise.
* gnu/services/networking.scm (tor-service): Likewise.

services: Update to use the dmd 0.2 API.

* gnu/services/avahi.scm (avahi-service)[start]: Wrap command in a
  list.
* gnu/services/dbus.scm (dbus-service)[start]: Likewise.
* gnu/services/ssh.scm (lsh-service): Likewise.
* gnu/services/base.scm (mingetty-service)[start]: Likewise.
  (nscd-service)[start]: Likewise.
  (syslog-service)[start]: Likewise.
  (guix-service)[start]: Likewise.
  (udev-service)[start]: Use 'exec-command' instead of 'execl'.
* gnu/services/xorg.scm (slim-service)[start]: Likewise, and use
  #:environment-variables.

system: Add a 'system?' field to user accounts.

* gnu/system/shadow.scm (<user-account>)[system?]: New field.
* gnu/system.scm (user-account->gexp): Add it.
* guix/build/activation.scm (add-user): Add #:system? parameter and
  honor it.
  (activate-users+groups): Handle the 'system?' part of user tuples.
  Pass it to 'add-user'.  Don't create PROFILE-DIR when SYSTEM? is
  true.
* gnu/services/dbus.scm (dbus-service): Add 'system?' field for
  "messagebus" account.
* gnu/services/base.scm (guix-build-accounts): Likewise.
* gnu/services/avahi.scm (avahi-service): Likewise.

Merge branch 'master' into core-updates

services: Add D-Bus and Avahi.

* gnu/services/dbus.scm: New file.
* gnu/services/avahi.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add them.