Follow the Linux kernel's lead on "fallthrough"

The pseudo keyword 'fallthrough' is defined such that case statement
blocks must end with any of these keywords:
 * break;
 * fallthrough;
 * continue;
 * goto <label>;
 * return [expression];
 *
 *  gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes

The macro is defined either as

  __attribute__((__fallthrough__))

or as

  do {} while (0)  /* fallthrough */

not including the semicolon.

This change implements the Linux kernel style and updates several locations
where "/*fallthrough*/ and /* FALLTHROUGH */ were not previously replaced.

Externally imported code such as libedit, libtommath and sqlite are
restored to their unaltered state.

Change-Id: I69db8167b0d5884f55d96d72de3059a0235a1ba3

lib/roken/dirent-test.c

@@ -28,7 +28,7 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  **********************************************************************/
-
+#include <config.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
@@ -148,7 +148,7 @@ int teardown_test(void)
 
         strcmp(dirname + len + 1 - sizeof(TESTDIR)/sizeof(char), TESTDIR) == 0) {
 
-        /* fallthrough */
+        fallthrough;
 
     } else {
         /* did we create the directory? */
@@ -162,7 +162,7 @@ int teardown_test(void)
                     fprintf(stderr, "Can't change to test directory. Aborting cleanup.\n");
                     return -1;
                 } else {
-                    /* fallthrough */
+                    fallthrough;
                 }
             } else {
                 return -1;