Invalid types for result.stdout|stderr|all

[ehmicky]

result.stdout is undefined when either:

• the buffer option is false
• the stdout option is inherit, ipc, ignore, a Node.js stream or an integer. Or an array with a single item with one of those values.

At the moment, result.stdout is always typed as either string or Uint8Array instead.

This problem applies also to result.stderr and result.all.

[ehmicky]

Done in #681.