maint: avoid warnings due to attribute warn_unused_result

maint: avoid warnings due to attribute warn_unused_result

Now that a (void) cast no longer suffices to ignore warnings from gcc
about uses of functions marked with the warn_unused_result attribute,
we need an alternative.  For the record, here's one of the ignorable
warnings: "copy.c:233: warning: ignoring return value of 'fchown',
declared with attribute warn_unused_result"
* bootstrap.conf (gnulib_modules): Import ignore-value.
* src/copy.c: Include "ignore-value.h".
(set_owner): Use ignore_value in place of "(void)" casts,
to ignore lchown and fchown failures.
* src/cp.c (re_protect): Likewise, to ignore lchown failure.
* src/remove.c (preprocess_dir): Remove unnecessary "(void)" cast.