Don't downcase system diagnostics' first letters

Don't downcase system diagnostics' first letters

* etc/NEWS: Document this.
* lisp/emacs-lisp/bytecomp.el (byte-compile-file):
* lisp/ffap.el (find-file-at-point):
* lisp/files.el (insert-file-1):
* lisp/net/ange-ftp.el (ange-ftp-barf-if-not-directory)
(ange-ftp-copy-file-internal):
* lisp/progmodes/etags.el (visit-tags-table):
* lisp/url/url-dav.el (url-dav-delete-directory, url-dav-delete-file)
(url-dav-directory-files):
Keep diagnostics consistent with system's.
* lisp/erc/erc-dcc.el (erc-dcc-server):
* lisp/ffap.el (ffap-machine-p):
Ignore case while comparing diagnostics.
* src/fileio.c (report_file_errno): Don't downcase, and simplify.
Fixes: bug#19642