cmd/setec: do not prompt for secret values when stdin is a pipe

[creachadair]

I debated whether to make an additional flag to specifically request this
behaviour, but since we already sniff the terminal for output I thought I'd
start with this.

Curiously, I noticed we also trim space off a file input, which is kinda OK if
we know the file is text, but could break binary input. We might want a flag to
override that in one direction or other too.

[creachadair]

Another option, though perhaps too subtle: if utf8.Valid(value) { value = bytes.TrimSpace(value) }

[creachadair]

Oh I need to update the help text too.