update from texinfo
tests: simplify porting to Solaris 10 /bin/sh

Some test cases in 'grep' need a shell that groks '$(';
export re_shell_ for their benefit.  Problem reported for 'grep'
by Dagobert Michelsen in <http://bugs.gnu.org/16380>.
* tests/init.sh (re_shell_): Export if it's used.
tests: fix export bug in previous patch

Problem reported by Jim Meyering.
* tests/init.sh (re_shell): New var, which is exported instead of
re_shell_.
relocatable-shell: Update suggested usage in maintainer documentation.

Instead of suggesting an inline usage of "sed", that would have to be
cut-and-pasted for every directory to be relocated, suggest a shell
function.

Make the example obviously an example, whereas previously it looked like
it might be literal text.

Thanks to Reuben Thomas <rrt@sc3d.org> for pointing out these issues.  See
http://lists.gnu.org/archive/html/bug-gnulib/2014-01/msg00039.html for
further context.