strerror-override: avoid bloating errno module

strerror-override: avoid bloating errno module

* modules/errno (Files, configure.ac): Move replacement strings...
* modules/strerror-override: ...to new module.
* modules/strerror (Depends-on): Add strerror-override.
* modules/strerror_r-posix (Depends-on): Likewise.
* MODULES.html.sh: Document new module.
Reported by Bruno Haible.

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