Made the `show` command more scripting friendly

Added the --available (-a) and the --name-only (-N) options to the
show to command to make the output more suitable for (shell)
scripting (Bash completion of packages comes to mind).

The --available (-a) option will list only the  available packages,
similar to the --installed and --platform options.

The --name-only (-N) option will only show package names in the
listing (omitting version and description).

Additionally changed the output formatting when limiting the
package list result to remove the hierarchy when only one type is
being showed. This facilitates parsing of a list of packages (for
example for shell scripting and completion).