-C/resume without "proc exec"

-C/resume without "proc exec"

ftp(1) has "proc exec" to run sh(1) on interactive ! commands and filenames
starting with "|";  this is orthogonal to continuing transfers using the
existing file size as offsets.

There seems to be no case where a) the argument is an URL, i.e. we pledge,
and b) a shell is spawned somehow, so avoid these promises when resuming.

bsd.port.mk(5) FETCH_CMD uses -C by default.

OK millert