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/base64.c

@@ -282,7 +282,7 @@ b64_pton(char const *src, unsigned char *target, size_t targsize)
 				return (-1);
 			ch = *src++;		/* Skip the = */
 			/* Fall through to "single trailing =" case. */
-			/* FALLTHROUGH */
+			fallthrough;
 
 		case 3:		/* Valid, means two bytes of info */
 			/*

winsup/cygwin/libc/fnmatch.c

@@ -193,7 +193,7 @@ fnmatch1(const char *pattern, const char *string, const char *stringstart,
 					return (FNM_NOMATCH);
 				pattern += pclen;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 		norm:
 			string += sclen;

winsup/cygwin/libc/ftw.c

@@ -88,7 +88,7 @@ ftw(const char *path, int (*fn)(const char *, const struct stat *, int),
 			break;
 		case FTS_DC:
 			errno = ELOOP;
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			error = -1;
 			goto done;

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)

winsup/cygwin/libc/nftw.c

@@ -105,7 +105,7 @@ nftw(const char *path, int (*fn)(const char *, const struct stat *, int,
 			break;
 		case FTS_DC:
 			errno = ELOOP;
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			error = -1;
 			goto done;

winsup/cygwin/libc/rexec.cc

@@ -179,7 +179,7 @@ next:
 
 	case DEFAULT:
 		usedefault = 1;
-		/* FALL THROUGH */
+		fallthrough;
 
 	case MACH:
 		if (!usedefault) {

winsup/cygwin/libc/strptime.cc

@@ -524,7 +524,7 @@ literal:
 
 		case 'k':	/* The hour (24-hour clock representation). */
 			LEGAL_ALT(0);
-			/* FALLTHROUGH */
+			fallthrough;
 		case 'H':
 			LEGAL_ALT(ALT_O);
 			bp = conv_num(bp, &tm->tm_hour, 0, 23, ALT_DIGITS);
@@ -532,7 +532,7 @@ literal:
 
 		case 'l':	/* The hour (12-hour clock representation). */
 			LEGAL_ALT(0);
-			/* FALLTHROUGH */
+			fallthrough;
 		case 'I':
 			LEGAL_ALT(ALT_O);
 			bp = conv_num(bp, &tm->tm_hour, 1, 12, ALT_DIGITS);