Fix handling of setup file wrt setting tags

* org.el (org-set-regexps-and-options-for-tags): Return a list
with tag-related variables.
(org-set-regexps-and-options): Append tags from a setup file
to the local tags of the file.
(org-agenda-prepare-buffers): Set tags from a setup file by
calling `org-set-regexps-and-options' when necessary.

Thanks to Anupam Sengupta for reporting this.
Merge branch 'maint'
Fix commit b83c03
Merge branch 'maint'