doc: adjust reference to info nodes in man pages

doc: adjust reference to info nodes in man pages

old form: coreutils '$cmd invocation'
new form: '(coreutils) $cmd invocation'

The old form erroneously referenced the node for the 'coreutils'
multi-call program.  Now that problematic node name was renamed
in commit v8.23-18-g72e470b, but the newer less ambiguous form
also has the advantage of working with the pinfo viewer for example.
Full discussion at http://bugs.gnu.org/18428

* man/local.mk: Adjust man page references to texinfo nodes.
* src/system.h: Adjust --help references to texinfo nodes.