emacs: Add interface for licenses.

* emacs/guix-main.scm (%license-param-alist): New variable.
  (license->sexp, find-licenses, license-entries): New procedures.
* emacs/guix-license.el (guix-license-get-entries)
  (guix-license-get-display, guix-license-insert-packages-button)
  (guix-license-insert-comment, guix-license-list-describe)
  (guix-license-list-show-packages): New procedures.
  (guix-licenses): New command.
* doc/emacs.texi (Emacs Licenses): New node.
  (Emacs Interface): Add it.
* doc/guix.texi (Top): Add it.
* NEWS: Mention new interface.

emacs: list: Support displaying URL links.

* emacs/guix-list.el (guix-list-url): New face.
  (guix-list-get-url): New procedure.

emacs: Add 'guix-packages-by-license' command.

* emacs/guix-main.scm (packages-by-license): New procedure.
(%patterns-makers): Add 'license' search type.
* emacs/guix-messages.el (guix-message-packages-by-license): New procedure.
(guix-messages): Use it.
* emacs/guix-ui-package.el (guix-packages-by-license): New command.
* doc/emacs.texi (Emacs Commands): Document it.

emacs: Buttonize licenses in "Package Info".

* emacs/guix-ui-package.el (guix-package-license): New button type.
  (guix-package-info-format, guix-output-info-format): Use it.
* doc/emacs.texi (Emacs Info buffer): Mention it.

emacs: Add 'guix-browse-license-url' command.

* emacs/guix-main.scm (%licenses): New variable.
(licenses, license-names, lookup-license, lookup-license-uri): New
procedures.
* emacs/guix-read.el (guix-license-names, guix-read-license-name): New
procedures.
* emacs/guix-license.el: New file.
(guix-lookup-license-url): New procedure.
(guix-browse-license-url): New command.
* emacs.am (ELFILES): Add new file.

emacs: Rename 'file-path' to 'file-name'.

* emacs/guix-info.el (guix-info-file-path): Rename to...
  (guix-info-file-name): ... this.
* emacs/guix-list.el (guix-list-file-path): Rename to...
  (guix-list-file-name): ... this.
  (guix-list-get-file-path): Rename to...
  (guix-list-get-file-name): ... this.
* emacs/guix-ui-generation.el (guix-generation-list-format): Adjust
  accordingly.
* emacs/guix-ui-system-generation.el (guix-system-generation-list-format):
  Likewise.
* NEWS: Mention faces renaming.

gnu: Add python-efl.

* gnu/packages/enlightenment.scm (python-efl, python2-efl): New variables.

gnu: cmocka: New variable.

* gnu/packages/check.scm (cmocka): New variable.

gnu: Add pixz.

* gnu/packages/compression.scm (pixz): New variable.

doc: Add policy about version numbers for VCS snapshots.

* doc/guix.texi (Version Numbers): Add paragraphs about VCS snapshot
version numbers.

doc: Remove unneeded backslashes.

* doc/guix.texi (Networking Services): Remove unneeded backslashes.

gnu: grue-hunter: Use 'https' URL.

* gnu/packages/grue-hunter.scm (grue-hunter): Use 'https' URL.

doc: 'base-initrd' has #:virtio? default to #t.

This is a followup to e26d507.

* doc/guix.texi (Initial RAM Disk): #:virtio? defaults to #t.

gnu: Add sortmerna.

* gnu/packages/bioinformatics.scm (sortmerna): New variable.

doc: Fix typo in section on offloading.

* doc/guix.texi (Daemon Offload Setup): Fix quoting typo in command.

gnu: calibre: Update to 2.49.0.

* gnu/packages/ebook.scm (calibre): Update to 2.49.0.

gnu: cross-gcc-arguments: Enable C++, disable building of libstdc++-v3.

* gnu/packages/cross-base.scm (cross-gcc-arguments)[arguments]: Disable
  building libstdc++-v3 and enable building C++ compiler.

gnu: Add Qtractor.

* gnu/packages/music.scm (qtractor): New variable.

gnu: yoshimi: Update to 1.3.8.2.

* gnu/packages/music.scm (yoshimi): Update to 1.3.8.2.

gnu: privoxy: Update to 3.0.24.

* gnu/packages/tor.scm (privoxy): Update to 3.0.24.