* guix/records.scm (%recutils-plus-rx): New variable.
(recutils->alist): Use it to read + lines.
* tests/records.scm ("recutils->alist with + lines"): New test.
* guix/records.scm (%recutils-field-rx): New variable.
(recutils->alist): New procedure, formerly known as `fields->alist'.
* guix/scripts/substitute-binary.scm (fields->alist): Use it.
* tests/records.scm ("recutils->alist"): New test.