ox: Simplify syntax for attributes

ox: Simplify syntax for attributes

* lisp/ox.el (org-export-read-attribute): Do not use `read' to read
  attributes.  Instead, extract keywords and values from it, which
  means each value will be a string when non-nil.
* contrib/lisp/ox-groff.el (org-groff-link--inline-image): Use new
  attribute syntax.  Small refactoring.
* lisp/ox-ascii.el (org-ascii-horizontal-rule): Use new attribute
  syntax.
* lisp/ox-beamer.el (org-beamer-plain-list): Use new attribute syntax.
* lisp/ox-html.el (org-html--textarea-block): Use new attribute
  syntax.
* lisp/ox-latex.el (org-latex--inline-image, org-latex--org-table,
  org-latex--math-table): Use new attribute syntax.
* lisp/ox-man.el (org-man-table--org-table): Use new attribute syntax.
  Small refactoring.
* lisp/ox-odt.el (org-odt-link--inline-image, org-odt-table-cell): Use
  new attribute syntax.
* testing/lisp/test-ox.el: Add tests.

This patch introduces two changes.  To begin with, it removes the need
for quoting and escaping characters.  Also, all non-nil values are
stored as strings.  As an exception "nil" is stored as nil.