Refactor stdio option for execa.node()

[ehmicky]

This is a refactoring of the stdio option handling for execa.node().

It also fixes one bug: undefined descriptors beyond the first three were converted to pipe. Those should be kept as undefined instead. See Node.js documentation:

null, undefined - Use default value. For stdio fds 0, 1, and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is 'ignore'.

This does not change behavior otherwise.

Some code comments and tests are added too.

@GMartigny