Backport the ksh2020 fix for timezone name determination

This fix for `printf '%T' now` on FreeBSD was written by
@krader1961. This is from https://github.com/att/ast/pull/591:

On FreeBSD calling tzset() does not guarantee the tzname array will
be correctly populated. On most systems that works but on FreeBSD you
have to call localtime() or a related function (e.g., ctime()).

This change also eliminates a potential, very small, memory leak due to
the strdup()'ed tznames not being freed.

src/lib/libast/tm/tminit.c:
 - Fix timezone name determination on FreeBSD and a memory leak.

NEWS

@@ -4,6 +4,10 @@ For full details, see the git log at:
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-06-13:
+- Fixed a timezone name determination bug on FreeBSD that caused the
+  output from `LC_ALL=C printf '%T' now` to print the wrong time zone name.
+
 2020-06-11:
 
 - Fixed a bug that caused running 'builtin -d' on a special builtin to

src/lib/libast/tm/tminit.c

@@ -247,10 +247,6 @@ tmlocal(void)
 		else if (e)
 			environ[0] = e;
 	}
-#endif
-#if _dat_tzname
-	local.standard = strdup(tzname[0]);
-	local.daylight = strdup(tzname[1]);
 #endif
 	tmlocale();
 
@@ -296,10 +292,8 @@ tmlocal(void)
 		 * POSIX
 		 */
 
-		if (!local.standard)
+		local.standard = strdup(tzname[0]);
-			local.standard = strdup(tzname[0]);
+		local.daylight = strdup(tzname[1]);
-		if (!local.daylight)
-			local.daylight = strdup(tzname[1]);
 	}
 	else
 #endif