Eliminate some mutation of program constants

Eliminate some mutation of program constants

* lisp/foldout.el:
* lisp/gnus/mml2015.el (mml2015-epg-key-image-to-string):
* lisp/international/ja-dic-cnv.el (skkdic-set-postfix)
(skkdic-set-prefix):
* lisp/international/quail.el (quail-insert-kbd-layout):
* lisp/mail/rmail.el (rmail-set-attribute-1):
* lisp/net/tramp.el (tramp-handle-memory-info):
* lisp/org/org-macs.el (org-replace-escapes):
Work on newly created objects, or use non-destructive operations.