packages: The result of 'bag-grafts' does not contain duplicates.

* guix/packages.scm (bag-grafts): Add call to 'delete-duplicates'.

grafts: Use dependency information from substitutes when possible.

This avoids starting derivation builds just for the sake of knowing the
references of their outputs, thereby restoring the expected behavior of
--dry-run when substitutes are available.

* guix/grafts.scm (non-self-references): Remove 'store' parameter, and
add 'references'.  Use it.  Update caller.
(references-oracle): New variable.
(cumulative-grafts): Add 'references' parameter and use it.  Update
callers.
(graft-derivation): Remove 'build-derivations' call.  Add call to
'references-oracle'.

store: Add 'references/substitutes'.

* guix/store.scm (references/substitutes): New procedure.
* tests/store.scm ("references/substitutes missing reference info")
("references/substitutes with substitute info"): New tests.

tests: Narinfos can specify an non-empty reference list.

* guix/tests.scm (derivation-narinfo): Add #:references and honor it.
(call-with-derivation-narinfo, call-with-derivation-substitute):
Likewise.
(with-derivation-narinfo, with-derivation-substitute): Add 'references'
keyword.

guix build: Set the build options early.

This fixes a bug whereby, with grafts leading to builds very early,
build options such as --substitute-urls would not be taken into account
yet.

Reported by Andreas Enge <andreas@enge.fr>.

* guix/scripts/build.scm (guix-build): Move 'opts' to the beginning.
Use 'with-store' instead of 'open-connection'.  Call
'set-build-options-from-command-line' right after 'with-store'.

gnu: letsencrypt, python-acme: Update to 0.4.2.

These packages should be updated together.

* gnu/packages/tls.scm (letsencrypt): Update to 0.4.2.
(python-acme, python2-acme): Update to 0.4.2.

gnu: python-cryptography, python-cryptography-vectors: Update to 1.2.3.

These packages should be updated together.

* gnu/packages/python.scm (python-cryptography): Update to 1.2.3.
(python-cryptography-vectors): Update to 1.2.3.

doc: Mention syslogd configuration.

* gnu/services/base.scm (syslog-service): Add xref to Inetutils in the
docstring.
(%default-syslog.conf): Export.
* doc/guix.texi (Base Services): Update accordingly.

lint: cve: Gracefully handle HTTP errors.

* guix/scripts/lint.scm (current-vulnerabilities*): New procedure.
(package-vulnerabilities): Use it.

gnu: gnome: Sync description and synopsis.

* gnu/packages/gnome.scm (gnome)[synopsis, description]: Use official
GNU blurb.

gnu: nano: Update to 2.5.3.

* gnu/packages/nano.scm (nano): Update to 2.5.3.

gnu: libgit2: Update to 0.23.4.

* gnu/packages/version-control.scm (libgit2): Update to 0.23.4.

gnu: findnewest: Update to 0.3.

* gnu/packages/version-control.scm (findnewest): Update to 0.3.

emacs: Use '@' to separate package names and version numbers.

This is a followup to commit 1b846da8c372bee78851439fd9e72b2499115e5a.

* emacs/guix-base.el (guix-package-name-specification): Use "@" instead
of "-".
* emacs/guix-main.scm (name+version->full-name): Likewise.
(package-inputs-names): Use 'make-package-specification' instead of
'package-full-name'.
(full-name->name+version): Update the docstring.
* emacs/guix-ui-package.el (guix-packages-by-name): Likewise.

emacs: hydra: Use '-' to separate job names and version numbers.

* emacs/guix-hydra.el (guix-hydra-job-name-specification): New procedure.
* emacs/guix-ui-package.el (guix-package-info-insert-systems)
(guix-package-list-latest-builds): Use it.

gnu: exim: Update to 4.86.2 [fixes CVE-2016-1531].

* gnu/packages/mail.scm (exim): Update to 4.86.2.

gnu: Add byzanz.

* gnu/packages/gnome.scm (byzanz): New variable.

doc: Improve Info categories.

* doc/guix.texi: Change main category to "System administration".  Fix
syntax of subnodes.  Add entries for 'guix pull' and 'guix gc'.  Add
"Emacs" category.

build: Use (guix grafts) as needed.

This is a followup to 7adf9b8.

* build-aux/check-final-inputs-self-contained.scm: Use (guix grafts).
* build-aux/check-available-binaries.scm: Likewise.

gnu: youtube-dl: Update to 2016.03.01.

* gnu/packages/video.scm (youtube-dl): Update to 2016.03.01.