Cygwin: Add 'fallthrough' pseudo keyword for switch/case use

This patch has been inspired by the Linux kernel patch

  294f69e662d1 compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

written by Joe Perches <joe AT perches DOT com> based on an idea from
Dan Carpenter <dan DOT carpenter AT oracle DOT com>.  The following text
is from the original log message:

Reserve the pseudo keyword 'fallthrough' for the ability to convert the
various case block /* fallthrough */ style comments to appear to be an
actual reserved word with the same gcc case block missing fallthrough
warning capability.

All switch/case blocks now should end in one of:

	break;
	fallthrough;
	goto <label>;
	return [expression];
	continue;

In C mode, GCC supports the __fallthrough__ attribute since 7.1,
the same time the warning and the comment parsing were introduced.

Cygwin-only: add an explicit -Wimplicit-fallthrough=5 to the build
flags.

winsup/cygwin/libc/msgcat.c

@@ -81,6 +81,7 @@ static pthread_rwlock_t		 rwlock = PTHREAD_RWLOCK_INITIALIZER;
 #else
 
 #include "../locale/setlocale.h"
+#include "winsup.h"
 #define SIZE_T_MAX __SIZE_MAX__
 #define _close close
 #define _open open
@@ -233,7 +234,7 @@ catopen(const char *name, int type)
 						break;
 					case '%':
 						++nlspath;
-						/* FALLTHROUGH */
+						fallthrough;
 					default:
 						if (pathP - path >=
 						    sizeof(path) - 1)