illumos gate

This issue is about both awk and awk_xpg4. Identified by gcc 7 build:

    awk4.c: In function 'dosub':
    awk4.c:247:3: error: this statement may fall through [-Werror=implicit-fallthrough=]
       awkerr(nomem);
       ^~~~~~~~~~~~~
    awk4.c:248:2: note: here
      default:
      ^~~~~~~
    awk4.c: In function 'f_split':
    awk4.c:387:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
       if (isstring(tnp->n_flags) && tnp->n_string==_null)
          ^
    awk4.c:389:2: note: here
      default:
      ^~~~~~~

    awk3.c: In function 'assign':
    awk3.c:270:3: error: this statement may fall through [-Werror=implicit-fallthrough=]
       awkerr(gettext("cannot assign to function \"%s\""),
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           left->n_name);
           ~~~~~~~~~~~~~
    awk3.c:273:2: note: here
      default:
      ^~~~~~~
    awk3.c: In function 'action':
    awk3.c:748:8: error: this statement may fall through [-Werror=implicit-fallthrough=]
         if (isstring(l->n_flags) &&
            ^
    awk3.c:751:4: note: here
        default:
        ^~~~~~~
    awk3.c:688:4: error: this statement may fall through [-Werror=implicit-fallthrough=]
        doend(act);
        ^~~~~~~~~~
    awk3.c:691:3: note: here
       case RETURN:
       ^~~~

    awk1.c: In function 'yylex':
    awk1.c:558:8: error: this statement may fall through [-Werror=implicit-fallthrough=]
         if (nbrace != 0)
            ^
    awk1.c:561:4: note: here
        default:
        ^~~~~~~
    awk1.c: In function 'lexid':
    awk1.c:797:4: error: this statement may fall through [-Werror=implicit-fallthrough=]
        ++inprint;
        ^~~~~~~~~
    awk1.c:798:3: note: here
       default:
       ^~~~~~~

    awk.h:57:18: error: this statement may fall through [-Werror=implicit-fallthrough=]
     #define FATAL ), error(1, errbuf)
                      ^~~~~~~~~~~~~~~~
    b.c:640:23: note: in expansion of macro 'FATAL'
            lastre, prestr FATAL;
                           ^~~~~
    b.c:642:2: note: here
      default:
      ^~~~~~~
    cc1: all warnings being treated as errors