Revert "system: Add a 'needed-for-boot?' field to 'mapped-device'."

This reverts commit 3b09332adf7ce8e976a4d117a62c586a53af04aa, which
turned out to be a bad idea because we need to have dependency
information between the device-mapping service and the file-system
service that uses it.
doc: Explain that 'file-system' must refer to mapped devices by file name.

* doc/guix.texi (File Systems): Clarify what to do when mapped devices
  are used.
daemon: Default to '--cores=0'.

Suggested by Deck Pickard <deck.r.pickard@gmail.com>.

* nix/nix-daemon/guix-daemon.cc (main): Set 'settings.buildCores' to 0.
* doc/guix.texi (Invoking guix-daemon): Adjust accordingly.
guix: scripts: Add GUIX_BUILD_OPTIONS environment handling.

* doc/guix.texi (Invoking guix build): Mention 'GUIX_BUILD_OPTIONS'.
* guix/scripts/archive.scm: (append args (environment-build-options)).
* guix/scripts/build.scm: Ditto.
* guix/scripts/environment.scm: Ditto.
* guix/scripts/package.scm: Ditto.
* guix/scripts/system.scm: Ditto.
* guix/ui.scm (environment-build-options): New function.
* guix/utils.scm (arguments-from-environment-variable): New function.
* tests/guix-build.sh: Add tests.
* test-env.in: Unset GUIX_BUILD_OPTIONS.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>
gnu: wpa-supplicant: Remove dependency on DBus.

* gnu/packages/admin.scm (wpa-supplicant): Remove dependency on DBUS.
gnu: Add git-flow.

* gnu/packages/version-control.scm (git-flow, shflags): New variables.
gnu: Add git-test-sequence.

* gnu/packages/version-control.scm (git-test-sequence): New variable.
gnu: cd-discid: Fix download location.

* gnu/packages/cdrom.scm (cd-discid): Fix download location.
gnu: ucommon: Update to 6.2.3.

* gnu/packages/telephony.scm (ucommon): Update to 6.2.3.
gnu: global: Update to 6.3.3.

* gnu/packages/global.scm (global): Update to 6.3.3.
gnu: help2man: Update to 1.46.4.

* gnu/packages/man.scm (help2man): Update to 1.46.4.
gnu: freeipmi: Update to 1.4.6.

* gnu/packages/freeipmi.scm (freeipmi): Update to 1.4.6.
gnu: libmicrohttpd: Update to 0.9.38.

* gnu/packages/gnunet.scm (libmicrohttpd): Update to 0.9.38.
gnu: ocrad: Update to 0.24.

* gnu/packages/ocrad.scm (ocrad): Update to 0.24.
gnu: parallel: Update to 20141122.

* gnu/packages/parallel.scm (parallel): Update to 20141122.
gnu: libtasn1: Update to 4.2.

* gnu/packages/gnutls.scm (libtasn1): Update to 4.2.
gnu: libgpg-error: Update to 1.17.

* gnu/packages/gnupg.scm (libgpg-error): Update to 1.17.
build-system/glib-or-gtk: Clean up whitespace.

* guix/build/glib-or-gtk-build-system.scm: M-x whitespace-cleanup.
build-system/glib-or-gtk: Use 'for-each' and 'cut' as appropriate.

* guix/build/glib-or-gtk-build-system.scm (wrap-all-programs): Use
  'for-each' instead of 'map' in for-effect contexts.  Use 'cut' instead
  of 'lambda' when appropriate.
build-system/glib-or-gtk: Correctly handle multiple-output packages.

* guix/build/glib-or-gtk-build-system.scm (wrap-all-programs): Move body
  to 'handle-output' inner procedure, and parametrize it by output name
  and directory.  Call it for each element of OUTPUTS.
  (compile-glib-schemas): Likewise.