gnu-maintenance: Fix file descriptor leak.

* guix/gnu-maintenance.scm (official-gnu-packages): Close the port
  returned by FETCH.

gnu-maintenance: Use 'http-fetch/cached' by default.

This speeds up operations like 'guix lint -c gnu-description'.

* guix/gnu-maintenance.scm (official-gnu-packages): Add 'fetch'
  parameter and honor it.  Default to 'http-fetch/cached'.

http-client: Add 'http-fetch/cached'.

* guix/utils.scm (cache-directory): New procedure.
* guix/http-client.scm (%http-cache-ttl): New variable.
  (http-fetch/cached): New procedure.

syscalls: Fix ABI mismatch for 'clone'.

Fixes <http://bugs.gnu.org/21694>.

* guix/build/syscalls.scm (clone): Change 'syscall' parameter types to
  LONG, UNSIGNED-LONG, or '*; make sure it has 6 parameters.  Adjust
  caller accordingly.

guix system: Add 'dmd-graph' command.

* guix/scripts/system.scm (dmd-service-node-label,
  dmd-service-node-type, export-dmd-graph): New procedures.
  (show-help): Add 'dmd-graph'.
  (guix-system)[parse-sub-command]: Likewise.
  Honor it.
* doc/guix.texi (Invoking guix system): Document it.
  (dmd Services): Add an illustration and explanation.
* doc/images/dmd-graph.dot: New file.
* doc.am (DOT_FILES): Add it.

guix system: Add 'extension-graph' command.

* guix/scripts/system.scm (service-node-label, service-node-type,
  export-extension-graph): New procedures.
  (guix-system)[parse-sub-command]: Add 'extension-graph'.
  Honor it.
  (show-help): Add 'extension-graph'.
* doc/guix.texi (Invoking guix system): Document it.
  (Service Composition): Add cross-reference.

graph: 'export-graph' takes a #:reverse-edges? parameter.

* guix/scripts/graph.scm (export-graph): Add #:reverse-edges? parameter
  and honor it.

build-system/gnu: dist-package: Use 'autoconf-wrapper'.

* guix/build-system/gnu.scm (dist-package): Use AUTOCONF-WRAPPER instead
  of AUTOCONF.

build-system/python: Memoize the results of 'package-with-python2'.

Fixes <http://bugs.gnu.org/21675>.
Reported by Cyril Roelandt <tipecaml@gmail.com>.

Before this change, the command:

  guix build python2-oslo.utils -n --no-substitutes

would take 17.5s.  After, it is down to 2.9s.
Likewise, the command:

  guix graph python2-bandit | grep python2-setuptools.*Helve| wc -l

would return 412 nodes before, all functionally equivalent, and returns
a single one now.

* guix/build-system/python.scm (package-with-explicit-python): Remove
  'p' parameter.  Change to return a one-argument memoizing procedure.
  (package-with-python2): Adjust accordingly.

Merge branch 'core-updates'

emacs: Add 'guix-devel-build-package-source'.

Suggested by Ludovic Courtès <ludo@gnu.org>.

* guix/scripts.scm (build-package-source): New procedure.
* emacs/guix-devel.el (guix-devel-build-package-source): New command.
  (guix-devel-keys-map): Add key binding for it.
* doc/emacs.texi (Emacs Development): Document it.

guix system: Add '--derivation'.

* guix/scripts/system.scm (perform-action): Add #:derivations-only?
  parameter and honor it.
  (show-help, %options): Add '--derivation'.
  (guix-system): Pass #:derivations-only? to 'perform-action'.
* tests/guix-system.sh: Test it.
* doc/guix.texi (Invoking guix system): Document it.

gexp: Add 'scheme-file'.

* guix/gexp.scm (<scheme-file>): New record type.
  (scheme-file, scheme-file-compiler): New procedures.
* tests/gexp.scm ("scheme-file"): New test.
* doc/guix.texi (G-Expressions): Document 'scheme-file'.

gexp: Add 'mixed-text-file'.

* guix/gexp.scm (mixed-text-file): New procedure.
* tests/gexp.scm ("mixed-text-file"): New test.
* doc/guix.texi (G-Expressions): Document it.

gexp: Add 'program-file'.

* guix/gexp.scm (<program-file>): New record type.
  (program-file, program-file-compiler): New procedures.
* tests/gexp.scm ("program-file"): New test.
* doc/guix.texi (G-Expressions): Document it.

gexp: Add 'computed-file'.

* guix/gexp.scm (<computed-file>): New record type.
  (computed-file, computed-file-compiler): New procedures.
* tests/gexp.scm ("lower-object, computed-file"): New test.
* doc/guix.texi (G-Expressions): Document 'computed-file'.

scripts: environment: Use system* instead of system.

This allows for direct program invokation without needing a shell to act
as a command interpreter.

* guix/scripts/environment.scm (%default-shell): New variable.
  (show-help): Adjust description.  Remove '--exec' reference.
  (%default-options): Use '%default-shell'.
  (%options): Adjust '--exec' to run command via the default shell.
  (parse-args): New procedure.
  (guix-environment): Use 'parse-args'.  Use 'system*' instead of
  'system'.
* tests/guix-environment.sh: Add test for '--' command invokation.
* doc/guix.texi ("Invoking guix environment"): Use new syntax.  Remove
  '--exec' documentation.

utils: Add split procedure.

* guix/utils.scm (split): New procedure.
* tests/utils.scm: Add tests.

lint: Export 'run-checkers'.

* guix/scripts/lint.scm (run-checkers): Export.  Make 'checkers'
  argument optional.

Merge branch 'master' into core-updates