echo: always process escapes when POSIXLY_CORRECT is set
* src/echo.c (main): Always enable backslash processing if
POSIXLY_CORRECT is set.
* tests/misc/echo.sh: Add (the first) test for the echo command.
* tests/local.mk: Reference the new test.
* tests/misc/printf.sh: Update a stale comment.
* doc/coreutils.texi (echo invocation). Mention that POSIXLY_CORRECT
now always enables backslash processing.
* NEWS: Mention the change in behavior.
Fixes https://bugs.gnu.org/32703
Issue identified by Eric Blake.