strerror: drop strerror_r dependency

strerror: drop strerror_r dependency

Since the errno module is responsible for introducing replacement
errno values, it should also be responsible for translating those
new values into strings.  And by moving the replacements into a
file managed by the errno, we can then break the dependency between
strerror and strerror_r, so that strerror no longer drags in
multi-threading modules required by strerror_r.

Tested on glibc with:

gl_cv_header_errno_h_complete=no gl_cv_func_working_strerror=no \
  gl_cv_func_strerror_r_works=no ./gnulib-tool --with-tests \
  --test strerror strerror_r-posix

* lib/strerror_r.c (strerror_r): Move gnulib replacement strings...
* lib/strerror-override.c (strerror_override): ...to new file.
* lib/strerror-override.h: Add prototype.
* lib/strerror-impl.h: Delete.
* lib/strerror.c (strerror): New implementation.
* modules/errno (Files): Add new files.
(configure.ac): Compile new file as appropriate.
* modules/strerror (Files): Drop unused file.
(Depends-on): Drop strerror_r-posix.
* MODULES.html.sh: Document strerror_r-posix.
Requested by Sam Steingold.

Signed-off-by: Eric Blake <eblake@redhat.com>