minor tz tuneups and portability fix

[Paul Eggert]

Here are some proposed very minor code tuneups for the tz code.

While accumulating these I noticed a portability bug in localtime.c;
the expression `(t < 0) ? 0 : ((time_t) 1 << bits)' yields undefined
results if time_t is unsigned, since `bits' is the number of bits in
a time_t in that case.  The patch below fixes this bug as part of the tuneups.

===================================================================
RCS file: RCS/tzfile.h,v
retrieving revision 1995.3
retrieving revision 1995.3.1.1
diff -c -r1995.3 -r1995.3.1.1
*** tzfile.h	1995/03/11 17:55:58	1995.3
--- tzfile.h	1996/01/15 06:22:59	1995.3.1.1
***************
*** 153,159 ****
  ** that will probably do.
  */
  
! #define isleap(y) ((((y) % 4) == 0 && ((y) % 100) != 0) || ((y) % 400) == 0)
  
  #ifndef USG
  
--- 153,159 ----
  ** that will probably do.
  */
  
! #define isleap(y) (((y) % 4) == 0 && (((y) % 100) != 0 || ((y) % 400) == 0))
  
  #ifndef USG
  
===================================================================
RCS file: RCS/difftime.c,v
retrieving revision 1994.8
retrieving revision 1994.8.1.1
diff -c -r1994.8 -r1994.8.1.1
*** difftime.c	1994/12/09 04:33:32	1994.8
--- difftime.c	1996/01/15 06:22:59	1994.8.1.1
***************
*** 43,51 ****
  	/*
  	** Repair delta overflow.
  	*/
! 	hibit = 1;
! 	while ((hibit <<= 1) > 0)
! 		continue;
  	/*
  	** The following expression rounds twice, which means
  	** the result may not be the closest to the true answer.
--- 43,49 ----
  	/*
  	** Repair delta overflow.
  	*/
! 	hibit = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
  	/*
  	** The following expression rounds twice, which means
  	** the result may not be the closest to the true answer.
***************
*** 65,74 ****
  	** This problem occurs only with very large differences.
  	** It's too painful to fix this portably.
  	** We are not alone in this problem;
! 	** many C compilers round twice when converting
  	** large unsigned types to small floating types,
  	** so if time_t is unsigned the "return delta" above
! 	** has the same double-rounding problem.
  	*/
  	return delta - 2 * (long_double) hibit;
  }
--- 63,72 ----
  	** This problem occurs only with very large differences.
  	** It's too painful to fix this portably.
  	** We are not alone in this problem;
! 	** some C compilers round twice when converting
  	** large unsigned types to small floating types,
  	** so if time_t is unsigned the "return delta" above
! 	** has the same double-rounding problem with those compilers.
  	*/
  	return delta - 2 * (long_double) hibit;
  }
===================================================================
RCS file: RCS/localtime.c,v
retrieving revision 1995.7
retrieving revision 1995.7.1.1
diff -c -r1995.7 -r1995.7.1.1
*** localtime.c	1995/10/30 15:32:46	1995.7
--- localtime.c	1996/01/15 06:22:59	1995.7.1.1
***************
*** 1163,1175 ****
  	tmp->tm_hour = (int) (rem / SECSPERHOUR);
  	rem = rem % SECSPERHOUR;
  	tmp->tm_min = (int) (rem / SECSPERMIN);
! 	tmp->tm_sec = (int) (rem % SECSPERMIN);
! 	if (hit)
! 		/*
! 		** A positive leap second requires a special
! 		** representation.  This uses "... ??:59:60" et seq.
! 		*/
! 		tmp->tm_sec += hit;
  	tmp->tm_wday = (int) ((EPOCH_WDAY + days) % DAYSPERWEEK);
  	if (tmp->tm_wday < 0)
  		tmp->tm_wday += DAYSPERWEEK;
--- 1163,1173 ----
  	tmp->tm_hour = (int) (rem / SECSPERHOUR);
  	rem = rem % SECSPERHOUR;
  	tmp->tm_min = (int) (rem / SECSPERMIN);
! 	/*
! 	** A positive leap second requires a special
! 	** representation.  This uses "... ??:59:60" et seq.
! 	*/
! 	tmp->tm_sec = (int) (rem % SECSPERMIN) + hit;
  	tmp->tm_wday = (int) ((EPOCH_WDAY + days) % DAYSPERWEEK);
  	if (tmp->tm_wday < 0)
  		tmp->tm_wday += DAYSPERWEEK;
***************
*** 1344,1358 ****
  	/*
  	** Calculate the number of magnitude bits in a time_t
  	** (this works regardless of whether time_t is
! 	** signed or unsigned, though lint complains if unsigned).
  	*/
! 	for (bits = 0, t = 1; t > 0; ++bits, t <<= 1)
! 		continue;
  	/*
  	** If time_t is signed, then 0 is the median value,
! 	** if time_t is unsigned, then 1 << bits is median.
  	*/
! 	t = (t < 0) ? 0 : ((time_t) 1 << bits);
  	for ( ; ; ) {
  		(*funcp)(&t, offset, &mytm);
  		dir = tmcomp(&mytm, &yourtm);
--- 1342,1355 ----
  	/*
  	** Calculate the number of magnitude bits in a time_t
  	** (this works regardless of whether time_t is
! 	** signed or unsigned).
  	*/
! 	bits = TYPE_BIT(time_t) - TYPE_SIGNED(time_t);
  	/*
  	** If time_t is signed, then 0 is the median value,
! 	** if time_t is unsigned, then 1 << (bits - 1) is median.
  	*/
! 	t = TYPE_SIGNED(time_t) ? 0 : ((time_t) 1 << (TYPE_BIT(time_t) - 1));
  	for ( ; ; ) {
  		(*funcp)(&t, offset, &mytm);
  		dir = tmcomp(&mytm, &yourtm);