Further removal of attempt to support floating-point time_t.

* localtime.c (timesub):
* zdump.c (my_localtime): Don't worry about time_t being floating-point.
* localtime.c (timesub): Avoid undefined behavior on integer
overflow when assigning time_t to int.
* zone.tab (America/Shiprock): Remove.

This unnecessary duplicate of America/Denver was confusing, as it
incorrectly implied that 'Mountain Time - Navajo' differed from
'Mountain Time' either now or in the past.  Clarify this in a
better way, by mentioning in the Arizona entry that it doesn't
include Navajo.
Add -t ZONETABTYPE option to 'tzselect'.

This is a backwards-compatible way of transitioning to a less
politically-charged approach to choosing TZ values.  A new time
zone table time.tab acts like zone.tab, except it omits TZ values
that are present only for political reasons.  tzselect has a new
"-t zonetabtype" option that lets users select which time zone table
to use, and installers can select the default table.  The zone.tab
file does not change, and the default tzselect table for now is
zone.tab.  This avoids the backward-compatibility concerns
expressed on the tz mailing list; see, for example,
the discussion surrounding Derick Rethans's comments in
<http://mm.icann.org/pipermail/tz/2013-August/019544.html>
* .gitignore: Add time.tab.
* Makefile (ZONETABTYPE): New macro.
(TABDATA): Add time.tab.
(all): Depend on $(TABDATA).
(install): Do not depend on $(TABDATA), since $(DATA) includes it.
Install time.tab too.
(time.tab): New rule.
(tzselect.tab): Substitute ZONETABTYPE.
* zone-time.awk: New file.
* tzselect.8: Document new options, and --help and --version while
we're at it.
* tzselect.ksh (ZONETABTYPE): New var.
Implement -t ZONETABTYPE option.
Check that no extra operands are given.
* zone.tab: Change first comment line, to distinguish the two
tables better.