Partly fixes <http://bugs.gnu.org/20217>.
* guix/store.scm (set-build-options): Change #:substitute-urls to
default to #f. Send the 'substitute-urls' pair only if
SUBSTITUTE-URLS is true.
* guix/scripts/build.scm (set-build-options-from-command-line): Do not
default to %DEFAULT-SUBSTITUTE-URLS for #:substitute-urls.
* guix/scripts/size.scm (%default-options): Remove 'substitute-urls'.
This removes all I/O, which obviously makes things faster.
* guix/scripts/size.scm (file-size, store-item-exists?): Remove.
(query-path-info*): New procedure.
(file-size*): Rename to...
(file-size): ... this; adjust caller. Use 'query-path-info*' instead of
'file-size'.
* guix/scripts/size.scm: New file.
* Makefile.am (MODULES): Add it.
(SCM_TESTS): Add tests/size.scm.
* doc.am (SUBCOMMANDS): Add 'size'.
* po/guix/POTFILES.in: Add guix/scripts/size.scm.
* tests/size.scm: New file.
* doc/guix.texi (Packages with Multiple Outputs): Add xref to "Invoking guix
size".
(Invoking guix size): New node.
(Invoking guix gc): Add index for "closure" and xref to the above.
* doc/contributing.texi (Submitting Patches): Use @enumerate for the check
list. Add item about 'guix size'.