pipecmd: don't close stderr/stdout/stdin in parent when &pipecmd_preserve

This was reported and fixed by @m-schmoock for stderr, but I decided to
rework to make it clearer and cover the other cases:

	https://github.com/ElementsProject/lightning/pull/2321/commits/67dad01549f876e5e620eeffe2f1e78c35bd0f03

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>