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

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.