Rewrite setup keywords initialization
* lisp/org-table.el (org-table-set-constants): Remove function.
* lisp/org.el (org-set-regexps-and-options): Rewrite function. Merge
it with `org-set-regexps-and-options-for-tags'.
(org-set-regexps-and-options-for-tags): Remove function
(org--setup-collect-keywords, org--setup-process-tags): New functions.
(org-mode): Remove `org-set-regexps-and-options-for-tags' call.
(org-agenda-prepare-buffers): Use optimized setup for tags in all
cases. Improve docstring.
(org-make-options-regexp): Make returned regexp more efficient.