Pledge once with or without "proc exec", not twice

Pledge once with or without "proc exec", not twice

Spotted while comparing ktraces between 'tar -z' and 'gzcat | tar -f-'.

Only the former runs, e.g. gzip(1), but the latter also pledges theses promises
just to pledge again immediately afterwards without them.

Make the calls mutually exclusive so 'tar -f-' et al. skip the first pledge
and thus never have "proc exec" to begin wth.

"looks good to me" mbuhl
OK millert