Add support for testing time_t types other than the system's.

This makes it easier to test on (say) Debian, even if we're
testing the time_t type on (say) NetBSD.  NetBSD uses 64-bit
time_t on 32-bit hosts, and this lets us test a NetBSD-style
implementation (32-bit 'long', 64-bit time_t) on a 32-bit Debian host.
* Makefile: Update comments to talk about TIME_T_FLOATING and time_tz.
Sort the comments.
* private.h (restrict): Define to empty with older compilers.
'restrict' is now needed, to define gmtime_r and localtime_r in
standard ways when time_tz is defined.
Make the following changes if time_tz is defined:
(sys_time, time): New static functions.  The former is the system
'time' function that applies to the system time_t, the latter
our function that applies to our time_t.
(time_t, ctime, ctime_r, difftime, gmtime, gmtime_r, localtime)
(localtime_r, mktime): Rename to tz_time_t, tz_ctime, etc.,
via macros.  Declare the renamed versions.
* zdump.8: Document new options -V, -t.
* zdump.c: Include private.h if time_tz is defined.
(INITIALIZE): Remove; no longer needed.
(absolute_min_time, absolute_max_time): Work even if time_t
is wider than intmax_t, which can be true with GCC and __int128_t.
Use the new TIME_T_FLOATING macro for this.
(usage): Document new flags.
(main): Support them.
Add tests for 32- and 64-bit signed and unsigned time_t.

* Makefile (TIME_T_ALTERNATIVES): New macro.
(check_time_t_alternatives, clean_misc): New rules.
(clean): Split out into clean_misc and removing tzpublic.
(public): Add check_time_t.