Add RefTeX support for non-file buffers

Add RefTeX support for non-file buffers

* lisp/textmodes/reftex.el (reftex--get-buffer-identifier)
(reftex--get-directory, reftex--abbreviate-name)
(reftex--get-basename, reftex--get-truename): New helper
functions that handle both files and buffer objects.
(reftex--remove-buffer-from-master-index): New helper function.
(reftex-tie-multifile-symbols): Support non-file buffers, using
the above.
(reftex-TeX-master-file): Return current buffer when no file.
(reftex-access-scan-info): Remove check requiring file buffers.
(reftex-access-parse-file, reftex-check-parse-consistency): Skip
for non-file buffers.
(reftex-select-external-document): Use new helper function.
(reftex-locate-file): Return buffer objects directly.
(reftex-get-file-buffer-force): Handle buffer objects and
special 'buffer:' strings.
* lisp/textmodes/reftex-global.el (reftex-create-tags-file): Add
error handling for non-file buffers.  Leave TAGS unsupported in
non-file buffers.
(reftex-find-duplicate-labels)
(reftex-isearch-switch-to-next-file): Use new helper functions
that handle both files and buffer objects.
(reftex-isearch-switch-to-next-file): Use equal rather than
string= to compare strings/buffers.
* lisp/textmodes/reftex-index.el (reftex-display-index)
(reftex-index-change-entry): Use new helper functions.
(reftex-index-visit-phrases-buffer): Add error handling for
non-file buffers.  Leave phrases unsupported in non-file
buffers.
* lisp/textmodes/reftex-parse.el (reftex-do-parse)
(reftex-parse-from-file): Add support for non-file buffers.
(reftex-all-document-files, reftex-where-am-I)
(reftex-notice-new): Use new helper functions.
* lisp/textmodes/reftex-ref.el (reftex-label-info-update):
Support non-file buffers.
(reftex-label-info, reftex-replace-prefix-escapes, reftex-label)
(reftex-replace-prefix-escapes, reftex-offer-label-menu): Use
new helper functions.
* lisp/textmodes/reftex-sel.el (reftex-insert-docstruct): Use
new helper function.
* lisp/textmodes/reftex-toc.el (reftex-toc)
(reftex-recenter-toc-when-idle): Support non-file buffers.
* lisp/textmodes/reftex-cite.el (reftex-bib-or-thebib)
(reftex-get-bibfile-list): Use new helper functions.
* lisp/textmodes/reftex-global.el
(reftex-save-all-document-buffers, reftex-ensure-write-access):
Ignore non-file buffers.
* lisp/textmodes/reftex-parse.el (reftex-all-document-files):
For non-file objects, do not apply relative path transformation.
* test/lisp/textmodes/reftex-tests.el
(reftex-all-used-citation-keys-buffer)
(reftex-renumber-simple-labels-buffer): New tests for operations
on non-file buffers.

RefTeX historically assumed that the buffers it operates on
visit files.  To handle non-file buffers:

Modify reftex-TeX-master-file so that it returns the buffer
object itself (as suggested by Stefan Monnier).  Modify each
caller to handle buffer objects, aided by some helper functions
added to reftex.el.  Replace 'string=' by 'equal' in places.

Use buffer-base-buffer, where appropriate, to handle indirect
buffers (file and non-file).

TAGS files and phrases buffers remain unsupported for non-file
buffers.

(cherry picked from commit 03fc0e0fd51990570759601fa7e64f1cc5b76932)