systemctl: filter shown units by their load state

systemctl: filter shown units by their load state

E.g. systemctl --all -t masked gives the list of masked units.

The -t/--type option is reused. This is possible because unit types
and unit load states are called differently, so it is possible to
distinguish what the user meant. Using the same option also means that
the interface is user for the user: less options to remember.