vm: Fix recently-introduced regression in 'operating-system-services' use.

* gnu/system/vm.scm (operating-system-build-gid): Adjust to new return
  type of 'operating-system-services' introduced in 217a5b8.

services: Add 'file-system-service'.

* gnu/services/base.scm (file-system-service): New procedure.
  (user-processes-service): Add 'requirements' parameter.
* gnu/services/dmd.scm (dmd-configuration-file): Use (guix build
  linux-initrd).
* guix/build/linux-initrd.scm (guix): Export 'check-file-system'.
* gnu/system.scm (file-union): New procedure.
  (essential-services): Use it.  Add that to the returned list.

services: Use (guix build syscalls) instead of util-linux.

* gnu/services/dmd.scm (dmd-configuration-file): Add derivations for
  the (guix build syscalls) module, and add that to the load path of
  dmd.conf.
* gnu/services/base.scm (root-file-system-service): Rewrite using the
  'sync' and 'mount' procedures.

Add (guix build syscalls).

* guix/build/syscalls.scm, tests/syscalls.scm: New files.
* Makefile.am (MODULES): Add guix/build/syscalls.scm.
  (SCM_TESTS): Add tests/syscalls.scm.
* guix/utils.scm (%libc-errno-pointer, errno): Remove; take from (guix
  build syscalls).

linux-initrd: Append to /etc/mtab.

* guix/build/linux-initrd.scm (mount-file-system): Open /etc/mtab in
  append mode.

system: Automatically add essential services.

* gnu/services/base.scm (%base-services): Remove calls to
  'host-name-service', 'user-processes-service', and
  'root-file-system-service'.
* gnu/system.scm (<operating-system>)[operating-system-services]: Rename
  to...
  [operating-system-user-services]: ... this.
  (essential-services, operating-system-services): New procedures.
  (operating-system-accounts, operating-system-etc-directory,
  operating-system-boot-script, operating-system-derivation): Adjust to
  new 'operating-system-services' return type.

gnu: fftw: Factor out OpenMPI dependency.

* gnu/packages/algebra.scm (fftw)[inputs,arguments]: Remove OpenMPI.
  (fftw-openmpi): New variable.

gnu: Add hwloc.

* gnu/packages/mpi.scm (hwloc): New variable.
  (openmpi): Add TODO comment.

gnu: Add pciutils.

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

gnu: Add numactl.

* gnu/packages/linux.scm (numactl): New variable.

gnu: openmpi: Fix typo in download uri

* gnu/packages/mpi.scm (openmpi)[origin]: Fix typo.

gnu: fftw: Add dependency on openmpi.

* gnu/packages/algebra.scm (fftw)[inputs,arguments]: Add OpenMPI.

gnu: fftw: Upgrade to 3.3.4

* gnu/packages/algebra.scm (fftw): Upgrade to 3.3.4.

services: Add service to cleanly unmount the root file system.

* gnu/services/base.scm (root-file-system-service,
  user-processes-service): New procedures.
  (mingetty-service, nscd-service, syslog-service, guix-service): Add
  requirement on 'user-processes'.
  (%base-services): Add (user-processes-service)
  and (root-file-system-service).
* gnu/services/xorg.scm (slim-service): Add requirement on
  'user-processes'.

linux-initrd: Don't leak /dev/console file descriptors.

* guix/build/linux-initrd.scm (switch-root): Simplify /dev/console
  code.  This fixes a bug where we would leak the IN and OUT file
  descriptors.

doc: Mention Kiselyov's work on "staging".

* doc/guix.texi (G-Expressions): Mention Oleg's work on "staging" in
  footnote.

git-download: Disable TLS certificate verification.

* guix/build/git.scm (git-fetch): Add 'setenv' call.

gnu: openmpi: Avoid "open source" verbiage

* gnu/packages/mpi.scm (openmpi)[synopsis,description]: Remove mention
  of "open source".

gnu: Add petsc-openmpi.

* gnu/packages/maths.scm (petsc-openmpi): New variable.
  (petsc-complex-openmpi): New variable.

gnu: petsc: Clean some more leaked chroot references

* gnu/packages/maths.scm (petsc)[arguments]: Scrub some
  config-generated header files before build.  Only remove files if
  they exist.