Namibia was on DST until 2017-10-24

While we're at it, use negative DST offsets in vanguard format,
since Namibia (like Ireland) used negative DST in winter.
* NEWS, africa (Namibia, Africa/Windhoek), ziguard.awk: Implement this.

NEWS

@@ -1,5 +1,15 @@
 News for the tz database
 
+Unreleased, experimental changes
+
+  Changes to past time stamps
+
+    For Namibia's transition from +01/+02 to plain +02, change the
+    transition date from 2017-09-03 at 03:00 to 2017-10-24 at 00:00.
+    This does not affect UTC offsets, only whether a DST regime was
+    in place.
+
+
 Release 2018d - 2018-03-22 07:05:46 -0700
 
   Briefly:

africa

@@ -992,9 +992,20 @@ Link Africa/Maputo Africa/Lusaka	# Zambia
 # the same time they would normally start DST, the first Sunday in September:
 # https://www.timeanddate.com/news/time/namibia-new-time-zone.html
 
+# From Paul Eggert (2017-04-05):
+# The official date of the time zone change was 2017-10-24.  See:
+# http://www.lac.org.na/laws/annoSTAT/Namibian%20Time%20Act%209%20of%202017.pdf
+
 # RULE	NAME	FROM	TO	TYPE	IN	ON	AT	SAVE	LETTER/S
+# The following has standard time in summer and negative daylight
+# saving time in winter.  It is for when negative SAVE values are used.
+# For more about this, see the comments in 'europe' for Rule Eire.
+#Rule	Namibia	1994	only	-	Mar	21	0:00	-1:00	+01
+#Rule	Namibia	1994	2017	-	Sep	Sun>=1	2:00	0	CAT
+#Rule	Namibia	1995	2017	-	Apr	Sun>=1	2:00	-1:00	+01
+# The following is for when SAVE values are always nonnegative.
 Rule	Namibia	1994	only	-	Mar	21	0:00	0	-
-Rule	Namibia	1994	2016	-	Sep	Sun>=1	2:00	1:00	S
+Rule	Namibia	1994	2017	-	Sep	Sun>=1	2:00	1:00	S
 Rule	Namibia	1995	2017	-	Apr	Sun>=1	2:00	0	-
 # Zone	NAME		GMTOFF	RULES	FORMAT	[UNTIL]
 Zone	Africa/Windhoek	1:08:24 -	LMT	1892 Feb 8
@@ -1003,7 +1014,11 @@ Zone	Africa/Windhoek	1:08:24 -	LMT	1892 Feb 8
 			2:00	1:00	SAST	1943 Mar 21  2:00
 			2:00	-	SAST	1990 Mar 21 # independence
 			2:00	-	CAT	1994 Mar 21  0:00
-			1:00	Namibia	WA%sT	2017 Sep  3  2:00
+# The next line is for when negative SAVE values are used.
+#			2:00	Namibia	%s	2017 Oct 24
+# The next line is for when SAVE values are always nonnegative.
+			1:00	Namibia	WA%sT   2017 Oct 24
+# End of daylight saving.
 			2:00	-	CAT
 
 # Niger

ziguard.awk

@@ -26,6 +26,7 @@ BEGIN {
 
 outfile != "main.zi" {
   in_comment = /^#/
+  uncomment = comment_out = 0
 
   # If this line should differ due to Ireland using negative SAVE values,
   # uncomment the desired version and comment out the undesired one.
@@ -37,11 +38,32 @@ outfile != "main.zi" {
     if ((Rule_Eire \
 	 || (Zone_Dublin_post_1968 && $(in_comment + 3) == "IST/GMT"))	\
 	== vanguard) {
-      sub(/^#/, "")
-    } else if (/^[^#]/) {
-      sub(/^/, "#")
+      uncomment = in_comment
+    } else {
+      comment_out = !in_comment
     }
   }
+
+  # If this line should differ due to Namibia using negative SAVE values,
+  # uncomment the desired version and comment out the undesired one.
+  Rule_Namibia = /^#?Rule[\t ]+Namibia[\t ]/
+  Zone_using_Namibia_rule \
+    = (zone == "Africa/Windhoek" && /^#?[\t ]+[12]:00[\t ]/ \
+       && $(in_comment + 2) == "Namibia")
+  if (Rule_Namibia || Zone_using_Namibia_rule) {
+    if (in_comment == vanguard) {
+      uncomment = in_comment
+    } else {
+      comment_out = !in_comment
+    }
+  }
+
+  if (uncomment) {
+    sub(/^#/, "")
+  }
+  if (comment_out) {
+    sub(/^/, "#")
+  }
 }
 
 # If a Link line is followed by a Zone line for the same data, comment