* etc/completion/bash/guix (_guix_complete_subcommand): New function.
(_guix_complete_option): Allow completion of subcommand options.
(_guix_complete): Use '_guix_complete_subcommand' for "system" and
"import".
This avoids spurious messages when pressing TAB.
* etc/completion/bash/guix (_guix_complete_available_package)
(_guix_complete_installed_package, _guix_complete_option)
(_guix_complete): Redirect stderr to /dev/null when running 'guix'.
* etc/completion/bash/guix (_guix_complete_available_package): Cache the list
of available packages in '_guix_available_packages' and use it.
(_guix_complete): Cache the list of subcommands in '_guix_subcommands' and
use it.
