zmore: rewrite to fix bugs and assume POSIX

zmore: rewrite to fix bugs and assume POSIX

Problem reported for Solaris 9 by Daniel in
<http://lists.gnu.org/archive/html/bug-gzip/2012-06/msg00007.html>.
Rather than figure out what exactly went wrong in Solaris 9
it was easier to rip out all the buggy compatibility and stty cruft.
* zmore.in: Don't use stty or trap; simply pipe the output to 'more'
and let it deal with signals and terminal control.
Use printf, not 'echo', to avoid problems with backslashes.
Don't assume ANS is not 's' in the environment.
Use a 'more'-style header instead of rolling our own style.
Paginate the header, too; the old behavior lost the header.
* NEWS, zmore.1: Document this.