illumos gate

issue found by gcc 7 build:

    listen.c:958:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
          error(E_SYS_ERROR, EXIT | NO_MSG);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    listen.c:959:5: note: here
         case POLLHUP:
         ^~~~
    listen.c:961:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
          error(E_SYS_ERROR, EXIT | NO_MSG);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    listen.c:962:5: note: here
         case POLLNVAL:
         ^~~~
    listen.c:964:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
          error(E_SYS_ERROR, EXIT | NO_MSG);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    listen.c:965:5: note: here
         case POLLPRI:
         ^~~~
    listen.c:967:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
          error(E_SYS_ERROR, EXIT | NO_MSG);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    listen.c:968:5: note: here
         case POLLOUT:
         ^~~~
    listen.c:970:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
          error(E_SYS_ERROR, EXIT | NO_MSG);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    listen.c:971:5: note: here
         default:
         ^~~~~~~
    listen.c: In function 'doevent':
    listen.c:1081:3: error: this statement may fall through [-Werror=implicit-fallthrough=]
       sys_error(E_POLL, EXIT);
       ^~~~~~~~~~~~~~~~~~~~~~~
    listen.c:1083:2: note: here
      case T_LISTEN:
      ^~~~

    nstoa.c:215:25: error: this statement may fall through [-Werror=implicit-fallthrough=]
         buf[len++] = '\v'; s++;
                            ~^~
    nstoa.c:217:4: note: here
        case '0':
        ^~~~

    cc1: all warnings being treated as errors