Add unreachable.spatch coccinelle recipe and run it in precheck CI phase

cocci/.gitignore

@@ -0,0 +1 @@
+/*.patch

cocci/unreachable.spatch

@@ -0,0 +1,6 @@
+@@
+@@
+
+    INSIST(0);
++   ISC_UNREACHABLE();
+    ... when != ISC_UNREACHABLE();

util/check-cocci

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+ret=0
+for spatch in cocci/*.spatch; do
+    patch="$(dirname "$spatch")/$(basename "$spatch" .spatch).patch"
+    spatch --sp-file="$spatch" --use-gitgrep --dir . --very-quiet > "$patch";
+    if [ "$(< "$patch" wc -l)" -gt "0" ]; then
+	cat "$patch"
+	ret=1
+    else
+	rm "$patch"
+    fi
+done
+
+exit $ret

util/copyrights

@@ -2511,6 +2511,7 @@
 ./bin/win32/BINDInstall/resource.h		X	2001,2005,2009,2018
 ./bind.keys					X	2009,2010,2011,2017,2018
 ./bind.keys.h					X	2009,2010,2011,2012,2014,2017,2018
+./cocci/unreachable.spatch			X	2018
 ./config.guess					X	1998,1999,2000,2001,2004,2009,2013,2018
 ./config.h.in					X	1998,1999,2000,2001,2002,2003,2004,2005,2006,2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017,2018
 ./config.h.win32				C	1999,2000,2001,2004,2006,2007,2008,2009,2011,2012,2013,2014,2015,2016,2017,2018
@@ -4189,6 +4190,7 @@
 ./util/branchsync.sh				SH	2013,2016,2018
 ./util/check-categories.sh			SH	2015,2016,2017,2018
 ./util/check-changes				PERL	2002,2004,2007,2012,2016,2018
+./util/check-cocci				X	2018
 ./util/check-includes.pl			PERL	2000,2001,2004,2007,2012,2016,2017,2018
 ./util/check-instincludes.sh			SH	2000,2001,2004,2007,2012,2016,2018
 ./util/check-pullups.pl				PERL	2001,2002,2003,2004,2007,2012,2016,2018