du, wc: merge improved --files0-from=F-related diagnostics

du, wc: merge improved --files0-from=F-related diagnostics

du gave a better diagnostic for one unusual case,
and wc gave a better diagnostic for a different one.
Now each diagnoses both unusual cases.
* src/du.c (main): Disallow '-' as file name when reading from stdin.
* src/wc.c (main): Give a better diagnostic for a zero-length file name.