linux-initrd: Build /dev/loop* nodes.

* guix/build/linux-initrd.scm (make-essential-device-nodes): Build
  /dev/loop[0-7].

gnu: imagemagick: Upgrade to 6.8.9-0.

* gnu/packages/imagemagick.scm (imagemagick): Upgrade to 6.8.9-0.

gnu: perl-io-tty: Upgrade to 1.11.

* gnu/packages/perl.scm (perl-io-tty): Upgrade to 1.11.

guix: download: Update imagemagick mirrors.

* guix/download.scm (%mirrors)[imagemagick]: Update and add the legacy
    subdirectory of the main site as a last resort.

build: Fix builds without --with-libgcrypt-prefix nor --with-libgcrypt-libdir.

Reported by Manolis Ragkousis <manolis837@gmail.com>.
Fixes a regression introduced in 14af289.

Before this commit, ./configure would leave LIBGCRYPT_PREFIX and
LIBGCRYPT_LIBDIR undefined, leading to LIBGCRYPT_LIBS="-L -lgcrypt" and
thus to a link failure.

* configure.ac: Make sure $LIBGCRYPT_PREFIX and $LIBGCRYPT_LIBDIR are
  never empty.

vm: Modularize build-side code.

* guix/build/install.scm (install-grub): Call 'error' if 'system*'
  returns non-zero.
* guix/build/vm.scm (initialize-partition-table): Make 'partition-size'
  a positional parameter.  Call 'error' when 'system*' returns
  non-zero'.
  (format-partition, initialize-root-partition): New procedures.
  (initialize-hard-disk): Use them.

vm: Remove misleading comment.

* guix/build/vm.scm (load-in-linux-vm): Remove misleading comment.

gnu: Add wpa_supplicant.

* gnu/packages/admin.scm (wpa-supplicant): New variable.

store: Flush the output when the build process emits '\r'.

This allows progress reports emitted by 'substitute-binary' to be
correctly displayed.

* guix/store.scm (%newlines): New variable.
  (process-stderr) <%stderr-next>: Flush (current-build-output-port)
  when S contains one of %NEWLINES.

Augment (guix).

* guix.scm (%public-modules): Add 'monads' and 'gexp'.

Add (gnu system file-systems).

This fixes a circular dependency between (gnu system) and (gnu system
linux-initrd), where the latter could end up being compiled before
'file-system-type' was defined as a macro.

* gnu/system.scm (<file-system>, %fuse-control-file-system,
  %binary-format-file-system): Move to...
* gnu/system/file-systems.scm: ... here.  New file.
* build-aux/hydra/demo-os.scm, gnu/system/linux-initrd.scm,
  gnu/system/vm.scm: Use it.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.

linux-initrd: Display a backtrace when the initial program fails.

* guix/build/linux-initrd.scm (boot-system): Add pre-unwind handler in
  'catch' form around 'primitive-load', and call 'format' and
  'display-backtrace' from there.

vm: Fix typo.

Regression introduced in e38e18f.

* guix/build/vm.scm (initialize-hard-disk)[partition]: Use a string.

gnu: Add ccache.

* gnu/packages/ccache.scm: New file.
* gnu/packages/patches/ccache-stdc-predef-test.patch: New patch.
* gnu-system.am (GNU_SYSTEM_MODULES): Add ccache.scm.
  (dist_patch_DATA): Add patch.

gnu: Edit synopses of petsc packages.

* gnu/packages/maths.scm (petsc) [synopsis]: Shorten.
  (petsc-complex,petsc-openmpi,petsc-complex-openmpi) [synopsis]:
  Repeat synopsis from petsc package with extensions.  [description]: Remove.

gnu: offlineimap: fix license.

* gnu/packages/mail.scm (offlineimap) [license]: Change to gpl2+.

gnu: Add mu.

* gnu/packages/mail.scm (mu): New variable.

gnu: Add offlineimap

* gnu/packages/mail.scm (offlineimap): New variable.

gnu: Add Linux kbd.

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

ui: Gracefully deal with zero-output derivations.

* guix/ui.scm (show-what-to-build)[built-or-substitutable?]: New
  procedure.  Check whether OUT is #f.
  Use it.
* tests/ui.scm ("show-what-to-build, zero outputs"): New test.