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In the first case:

    pio.c: In function ‘main’:
    pio.c:355:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
        usage(0);
        ^~~~~~~~
    pio.c:356:3: note: here
       case 'm':
       ^~~~

The fallthrough is not intended because `usage()` never returns (it calls
`exit` unconditionally). Annotate as `noreturn` so the compiler realises this.

In the second case:

    fexc.c: In function ‘main’:
    fexc.c:312:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
       filename[1] = argv[optind+1]; /* out */
       ~~~~~~~~~~~~^~~~~~~~~~~~~~~~
    fexc.c:313:2: note: here
      case 1:
      ^~~~

The fallthrough appears to be intended (the two argument case is a superset of
the one argument case). Annotate with a comment which tells the compiler this
is intended.
Signed-off-by: Ian Campbell <ijc@hellion.org.uk>