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'.

doc: Add xrefs to the lsh manual.

Suggested by Adam Pribyl <pribyl@lowlevel.cz>.

* gnu/services/ssh.scm (lsh-service): Update docstring with xrefs to
  the lsh manual.  Fix typos.
* doc/guix.texi (Networking Services): Synchronize.

Merge branch 'master' into core-updates

services: lsh: Fix activation script.

* gnu/services/ssh.scm (lsh-service)[activate]: Import (guix build utils)
  for 'mkdir-p'.

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.

services: Add lshd service.

* gnu/services/ssh.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.