org-table: introduce an upper bound on `org-table-convert-region-max-lines'.
* lisp/org-table.el (org-table-convert-region-max-lines): New
defcustom.
(org-table-convert-region): Use it.
This is useful primarily for babel results. If a large table-like
object is returned by a code block, this function will become bogged
down in trying to read it, and hang emacs (necessitating a C-g). This
situation most commonly arises when a :results none header has been
omitted. With the patch, the user will not experience a hang, but
rather an error message.