cat: extend --show-ends to show \r\n as ^M$

- \r\n is common a line end combination
  - catting such a file without options causes it to display normally
  - overwriting the first char with $, loses info

* src/cat.c (cat): Convert \r preceeding a \n to ^M.
* tests/misc/cat-E.sh: New test.
* tests/local.mk: Reference new test.
* tests/misc/cat-proc.sh: Fix typo.
* doc/coreutils.texi (cat invocation): Mention the new behavior.
* NEWS: Mention the improvement.

tests/local.mk

@@ -279,6 +279,7 @@ all_tests =					\
   tests/misc/wc-nbsp.sh				\
   tests/misc/wc-parallel.sh			\
   tests/misc/wc-proc.sh				\
+  tests/misc/cat-E.sh				\
   tests/misc/cat-proc.sh			\
   tests/misc/cat-buf.sh				\
   tests/misc/cat-self.sh			\

tests/misc/cat-E.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+# Copyright (C) 2021 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ cat
+
+# \r followed by \n is displayed as ^M$
+# Up to and including 8.32 the $ would have displayed at the start of the line
+# overwriting the first character
+printf 'a\rb\r\nc\n\r\nd\r' > 'in' || framework_failure_
+printf 'a\rb^M$\nc$\n^M$\nd^M' > 'exp' || framework_failure_
+
+cat -E 'in' > out || fail=1
+
+compare exp out || fail=1
+
+Exit $fail

tests/misc/cat-proc.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-# Ensure that cat -E produces same output as cat, module '$'s,
+# Ensure that cat -E produces same output as cat, modulo '$'s,
 # even when applied to a file in /proc.
 
 # Copyright (C) 2006-2021 Free Software Foundation, Inc.