org-pcomplete: Modify completion for buffer keywords

org-pcomplete: Modify completion for buffer keywords

* lisp/org-pcomplete.el (pcomplete/org-mode/file-option): Collect
  valid keywords for completion without requiring the whole export
  framework.
(pcomplete/org-mode/file-option/options): Rewrite using new export
  framework.  Only complete up to the colon.
(pcomplete/org-mode/file-option/x): Removed macro.
(pcomplete/org-mode/file-option/title,
 pcomplete/org-mode/file-option/author,
 pcomplete/org-mode/file-option/email,
 pcomplete/org-mode/file-option/date): Removed functions.
(pcomplete/org-mode/file-option/infojs_opt): New function.

Completion for some keywords was removed because it provided only the
default value, which is the same as not writing the keyword at all.  In
other words, a user would almost always want to modify the completed
value, which makes the completion moot.