gnu: mpv: Fix build by using pkg-config for smbclient.

This is a followup to commit 710964d.

* gnu/packages/video.scm (mpv)[arguments]: Remove 'patch-wscript' phase.

gnu: agg: Update URLs.

* gnu/packages/graphics.scm (agg)[source, home-page]: Add up-to-date
URLs.

gnu: Use #:prefix when importing (guix licenses).

This works around a problem with #:hide, whereby Guile would report
'zlib', 'openssl', and/or 'expat as colliding bindings at run time
despite the #:hide.

* gnu/packages/linux.scm: Use #:prefix license for (guix licenses).
* gnu/packages/haskell.scm: Likewise.

gnu: Do not #:select from (gnu packages …) modules.

This is to avoid <http://bugs.gnu.org/15540>.

* gnu/packages/bittorrent.scm: Do not #:select from (gnu packages
compression).
* gnu/packages/dictionaries.scm: Likewise.
* gnu/packages/disk.scm: Likewise.
* gnu/packages/gimp.scm: Likewise.
* gnu/packages/haskell.scm: Likewise.

gnu: key-mon: Remove unnecessary propagated inputs.

* gnu/packages/key-mon.scm (key-mon)[propagated-inputs]: Rename to...
[inputs]: ... this.  Propagated was unnecessary since the 'key-mon'
program is wrapped.

gnu: python2-pygtk: Add "doc" output.

* gnu/packages/gtk.scm (python2-pygtk)[outputs]: New field.
[arguments]: Add 'set-gtk-doc-directory' phase.

gnu: python2-pygtk: Use 'modify-phases'.

* gnu/packages/gtk.scm (python2-pygtk)[arguments]: Use 'modify-phases'.

gnu: gimp: Add "doc" output.

* gnu/packages/gimp.scm (gimp)[outputs, arguments]: New fields.

gnu: goffice: Add "doc" output.

* gnu/packages/gnome.scm (goffice)[outputs, arguments]: New fields.
(goffice-0.8)[arguments]: Append arguments from GOFFICE.

gnu: Add dovecot service

* gnu/services/mail.scm: New file.
(&dovecot-configuration-error, dovecot-configuration-error?)
(dovecot-service, dovecot-configuration, dict-configuration)
(passdb-configuration, userdb-configuration)
(unix-listener-configuration, fifo-listener-configuration)
(inet-listener-configuration, service-configuration)
(protocol-configuration, plugin-configuration, mailbox-configuration)
(namespace-configuration, opaque-dovecot-configuration): New public
variables.

* gnu-system.am (GNU_SYSTEM_MODULES): Add (gnu services mail).

* doc/guix.texi (Mail Services): New node.

gnu: dovecot: Add linux-pam to inputs.

* gnu/packages/mail.scm (dovecot): Add linux-pam to inputs, enabling the
  PAM auth driver.

gnu: sdl-image: Add libwebp input.

* gnu/packages/sdl.scm (sdl-image)[propagated-inputs]: Add libwebp.

gnu: Add sdl2-ttf.

* gnu/packages/sdl.scm (sdl2-ttf): New variable.

gnu: Add sdl2-mixer.

* gnu/packages/sdl.scm (sdl2-mixer): New variable.

gnu: Add sdl2-image.

* gnu/packages/sdl.scm (propagated-inputs-with-sdl2): New procedure.
(sdl2-image): New variable.

gnu: samba: Update to 4.3.2.

* gnu/packages/samba.scm (samba): Update to 4.3.2.  Add more inputs.
[arguments]: Remove #:modules and #:imported-modules.  Remove 'chdir'
and 'add-lib-to-runpath' from #:phases.  Use 'modify-phases' syntax.

gnu: Add ldb.

* gnu/packages/samba.scm (ldb): New variable.

gnu: Add tevent.

* gnu/packages/samba.scm (tevent): New variable.

gnu: talloc: Update to 2.1.5.

* gnu/packages/samba.scm (talloc): Update to 2.1.5.
[arguments]: Modify wscript to skip the 'magic_differs' test.

gnu: talloc: Use 'modify-phases' syntax.

* gnu/packages/samba.scm (talloc)[arguments]: Use 'modify-phases' syntax.