tzselect: port to /bin/sh

Problem reported by Patrick 'P. J.' McDermott in
<http://mm.icann.org/pipermail/tz/2013-October/020441.html>.
This code is quite a bit different from what he proposed.
* tzselect.ksh: Rewrite so that it should work with /bin/sh on
common platforms.  For portability to Solaris 9 /bin/sh, use
`...`, not $(...), and avoid $((...)).
(doselect): New function.  Use this instead of plain 'select'.
Callers no longer need to worry whether it sets the var to empty.
* Makefile, NEWS: Document this.
tzselect: work around an old BusyBox awk bug

Patch by Patrick 'P. J.' McDermott in
<http://mm.icann.org/pipermail/tz/2013-October/020444.html>.
* tzselect.ksh: Replace awk -F options with FS assignments.
Before version 1.21.0, BusyBox awk didn't unescape the argument to
the -F option.  As a result, tzselect couldn't parse tables with
such versions of BusyBox awk.  See
<https://bugs.busybox.net/show_bug.cgi?id=5126> and
<http://git.busybox.net/busybox/commit?id=ea664dd>.
* NEWS: Document this.
* .gitignore: Ignore *.a files.