string: Simplify strerror_r

Use snprintf instead of mempcpy plus itoa_word and remove unused
definitions.  There is no potential for infinite recursion because
snprintf only use strerror_r for the %m specifier.

Checked on x86-64-linux-gnu, i686-linux-gnu, powerpc64le-linux-gnu,
and s390x-linux-gnu.

Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>