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/krb5/send_to_kdc.c

@@ -1192,8 +1192,7 @@ krb5_sendto_context(krb5_context context,
 		break;
 	    }
 	    action = KRB5_SENDTO_KRBHST;
-	    /* FALLTHROUGH */
-            fallthrough
+            fallthrough;
 	case KRB5_SENDTO_KRBHST:
 	    if (ctx->krbhst == NULL) {
 		ret = krb5_krbhst_init_flags(context, realm, type,
@@ -1215,8 +1214,7 @@ krb5_sendto_context(krb5_context context,
 		handle = heim_retain(ctx->krbhst);
 	    }
 	    action = KRB5_SENDTO_TIMEOUT;
-	    /* FALLTHROUGH */
-            fallthrough
+            fallthrough;
 	case KRB5_SENDTO_TIMEOUT:
 
 	    /*