gadgetd: indicate failure on unknown command line option

Instead of returning zero exit code when unknown option is passed,
exit with non-zero exit code.
gadgetd: scrap return value of parse_cmdline function

parse_cmdline function always returns zero.  Indeed the caller never
uses its return value.  Just drop it.
gadgetd: use printf(…) instead of fprintf(stdout, …)

Using printf instead of fprintf with stdout saves one line and makes
the code more concise.