Relax -C pledge to unbreak shelling out in interactive mode

Relax -C pledge to unbreak shelling out in interactive mode

r1.69 introduced -C in 2008 "to continue multiple transfers";
'ftp -C ftp://ftp.eu.openbsd.org/' lands in "ftp> " and turns "mget"
into "reget" by default.

r1.139 -C/resume without "proc exec" thusly was too strict.
Instead, now after recent cleanups/tweaks, prevent execution with -o.

OK millert