string-equal-ignore-case: new function

* lisp/cedet/semantic/complete.el (semantic-collector-calculate-completions):
  Use `string-prefix-p' instead of explicit `compare-strings'.
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns):
  Add `string-equal-ignore-case'.
* lisp/emacs-lisp/cl-extra.el (cl-equalp): Use `string-equal-ignore-case'.
* lisp/emacs-lisp/shadow.el (load-path-shadows-find): Likewise.
* lisp/emacs-lisp/shortdoc.el (string): Add `string-equal-ignore-case'.
* lisp/files.el (file-truename): Use `string-equal-ignore-case'.
(file-relative-name): Likewise.
* lisp/gnus/gnus-art.el (article-hide-boring-headers):
  Use `string-equal-ignore-case' instead of `gnus-string-equal'.
* lisp/gnus/gnus-util.el (gnus-string-equal):
  Remove, use `string-equal-ignore-case' instead.
* lisp/international/mule-cmds.el (describe-language-environment):
  Use `string-equal-ignore-case'.
(locale-charset-match-p): Likewise.
* lisp/man.el (Man-softhyphen-to-minus): Use `string-prefix-p'.
* lisp/minibuffer.el (completion--string-equal-p):
  Remove, use `string-equal-ignore-case' instead.
(completion--twq-all): Use `string-equal-ignore-case'.
(completion--do-completion): Likewise.
* lisp/net/browse-url.el (browse-url-default-windows-browser):
  Use `string-prefix-p' instead of explicit `compare-strings'.
* lisp/org/ob-core.el (org-babel-results-keyword):
  Use `string-equal-ignore-case' instead of explicit `compare-strings'.
(org-babel-insert-result): Likewise.
* lisp/org/org-compat.el (string-equal-ignore-case):
  Define unless defined already.
(org-mode-flyspell-verify): Use `string-equal-ignore-case'.
* lisp/org/org-lint.el (org-lint-duplicate-custom-id): Likewise.
* lisp/org/ox.el (org-export-resolve-radio-link): Use
  `string-equal-ignore-case' and `string-clean-whitespace'.
* lisp/progmodes/flymake-proc.el
  (flymake-proc--check-patch-master-file-buffer):
  Use `string-prefix-p' instead of explicit `compare-strings'.
* lisp/progmodes/idlwave.el (idlwave-class-or-superclass-with-tag):
  Use `string-equal-ignore-case' instead of explicit `compare-strings'.
* lisp/subr.el (member-ignore-case): Use `string-equal-ignore-case'.
(string-equal-ignore-case): Compare strings ignoring case.
* lisp/textmodes/bibtex.el (bibtex-string=): Remove.
(bibtex-format-entry, bibtex-font-lock-url, bibtex-autofill-entry)
(bibtex-print-help-message, bibtex-validate, bibtex-validate-globally)
(bibtex-clean-entry, bibtex-completion-at-point-function, (bibtex-url):
  Use `string-equal-ignore-case' instead of `bibtex-string='.
* lisp/textmodes/sgml-mode.el (sgml-get-context):
  Use `string-equal-ignore-case' instead of explicit `compare-strings'.
(sgml-calculate-indent): Likewise
* test/lisp/subr-tests.el (string-comparison-test):
  Add tests for `string-equal-ignore-case'.

lisp/emacs-lisp/shortdoc.el

@@ -243,6 +243,8 @@ A FUNC form can have any number of `:no-eval' (or `:no-value'),
   "Predicates for Strings"
   (string-equal
    :eval (string-equal "foo" "foo"))
+  (string-equal-ignore-case
+   :eval (string-equal-ignore-case "foo" "FOO"))
   (eq
    :eval (eq "foo" "foo"))
   (eql