Add %define lr.keep_unreachable_states.

* NEWS (2.3a+): Mention it in the entry for unreachable state removal.
* doc/bison.texinfo (Decl Summary): Mention it in the %define entry.
* src/main.c (main): Implement it.
* tests/conflicts.at (Unreachable States After Conflict Resolution):
Extend to test it, and fix a typo.

NEWS

@@ -3,8 +3,9 @@ Bison News
 
 Changes in version 2.3a+ (????-??-??):
 
-* Previously, Bison sometimes generated parser tables with states that were
-  unreachable due to conflicts in predecessor states.  Bison now:
+* Previously, Bison sometimes generated parser tables containing unreachable
+  states.  A state can become unreachable during conflict resolution if Bison
+  disables a shift action leading to it from a predecessor state.  Bison now:
 
     1. Removes unreachable states.
 
@@ -15,6 +16,13 @@ Changes in version 2.3a+ (????-??-??):
     3. For any rule used only in such states, Bison now reports the rule as
        "never reduced because of conflicts".
 
+  This feature can be disabled with the following directive:
+
+    %define lr.keep_unreachable_states
+
+  See the %define entry in the `Bison Declaration Summary' in the Bison manual
+  for further discussion.
+
 * When instructed to generate a `.output' file including lookahead sets
   (using `--report=lookahead', for example), Bison now prints each reduction's
   lookahead set only next to the associated state's one item that (1) is