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/passwd.cc

@@ -381,27 +381,27 @@ pg_ent::getent (void)
     {
     case rewound:
       state = from_cache;
-      /*FALLTHRU*/
+      fallthrough;
     case from_cache:
       if (nss_db_enum_caches ()
 	  && (entry = enumerate_caches ()))
 	return entry;
       state = from_file;
-      /*FALLTHRU*/
+      fallthrough;
     case from_file:
       if (from_files
 	  && nss_db_enum_files ()
 	  && (entry = enumerate_file ()))
 	return entry;
       state = from_builtin;
-      /*FALLTHRU*/
+      fallthrough;
     case from_builtin:
       if (from_db
 	  && nss_db_enum_builtin ()
 	  && (entry = enumerate_builtin ()))
 	return entry;
       state = from_local;
-      /*FALLTHRU*/
+      fallthrough;
     case from_local:
       if (from_db
 	  && nss_db_enum_local ()
@@ -410,7 +410,7 @@ pg_ent::getent (void)
 	  && (entry = enumerate_local ()))
 	return entry;
       state = from_sam;
-      /*FALLTHRU*/
+      fallthrough;
     case from_sam:
       if (from_db
 	  && nss_db_enum_local ()
@@ -422,14 +422,14 @@ pg_ent::getent (void)
 	  && (entry = enumerate_sam ()))
 	return entry;
       state = from_ad;
-      /*FALLTHRU*/
+      fallthrough;
     case from_ad:
       if (cygheap->dom.member_machine ()
 	  && from_db
 	  && (entry = enumerate_ad ()))
 	return entry;
       state = finished;
-      /*FALLTHRU*/
+      fallthrough;
     case finished:
       break;
     }
@@ -673,7 +673,7 @@ pw_ent::enumerate_caches ()
 	}
       cnt = 0;
       max = 1;
-      /*FALLTHRU*/
+      fallthrough;
     case 1:
       if (from_files)
 	{
@@ -684,7 +684,7 @@ pw_ent::enumerate_caches ()
 	}
       cnt = 0;
       max = 2;
-      /*FALLTHRU*/
+      fallthrough;
     default:
       if (from_db)
 	{