illumos gate

Issues found by gcc 7 build:

    sed0.c: In function 'fcomp':
    sed0.c:246:5: error: this statement may fall through [-Werror=implicit-fallthrough=]
         comperr("Unrecognized command: %s");
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    sed0.c:248:4: note: here
        case '!':
        ^~~~

    stty.c: In function 'main':
    stty.c:159:4: error: this statement may fall through [-Werror=implicit-fallthrough=]
        exit(0);
        ^~~~~~~
    stty.c:160:3: note: here
       case 'h':
       ^~~~
    stty.c:162:4: error: this statement may fall through [-Werror=implicit-fallthrough=]
        exit(0);
        ^~~~~~~
    stty.c:163:3: note: here
       default:
       ^~~~~~~

    tset.c: In function 'main':
    tset.c:560:18: error: this statement may fall through [-Werror=implicit-fallthrough=]
         Specialerase = YES;
                      ^
    tset.c:563:4: note: here
        case 'e': /* erase character */
        ^~~~
    tset.c:829:4: error: this statement may fall through [-Werror=implicit-fallthrough=]
        exit(-1);
        ^~~~~~~~
    tset.c:831:3: note: here
       case 0:
       ^~~~

    cc1: all warnings being treated as errors