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/hx509/req.c

@@ -1046,7 +1046,7 @@ authorize_feat(hx509_request req, abitstring a, size_t n, int idx)
     switch (ret) {
     case 0:
         req->nauthorized++;
-        /*fallthrough*/
+        fallthrough;
     case -1:
         return 0;
     default:
@@ -1063,7 +1063,7 @@ reject_feat(hx509_request req, abitstring a, size_t n, int idx)
     switch (ret) {
     case 0:
         req->nauthorized--;
-        /*fallthrough*/
+        fallthrough;
     case -1:
         return 0;
     default:
@@ -1245,7 +1245,7 @@ san_map_type(GeneralName *san)
             if (der_heim_oid_cmp(&san->u.otherName.type_id, map[i].oid) == 0)
                 return map[i].type;
     }
-        /*fallthrough*/
+        fallthrough;
     default:                               return HX509_SAN_TYPE_UNSUPPORTED;
     }
 }
@@ -1360,7 +1360,7 @@ hx509_request_get_san(hx509_request req,
     case HX509_SAN_TYPE_REGISTERED_ID:
         return der_print_heim_oid(&san->u.registeredID, '.', out);
     case HX509_SAN_TYPE_XMPP:
-        /*fallthrough*/
+        fallthrough;
     case HX509_SAN_TYPE_MS_UPN: {
         int ret;