Fix fd-specific options

[ehmicky]

Fixes #930.

Users can now set different values for stdout and stderr for a few options like verbose.

await execa(..., {verbose: {stdout: 'full', stderr: 'none'}})

When users pass things like stdout + all, the priority order is currently undefined.

await execa(..., {verbose: {stdout: 'full', all: 'none'}})

This PR defines a clearer priority order: stdout/stderr > fd* > all.