org-export: Internal modification to in-buffer options retrieval
* contrib/lisp/org-export.el (org-export-get-environment): Renamed
function from org-export-collect-options. Also make arguments
optionals.
(org-export-as): Apply renaming. Refactor code a bit.
(org-export-get-inbuffer-options): Remove string argument from
signature. Backend is now optional. Also, check if option match is
really a keyword.
(org-export-get-global-options, org-export-parse-option-keyword): Make
backend argument optional.