Add homed support
This integrates AccountsService w/ systemd-homed.
AccountsService can now:
- Enumerate systemd-homed users
- Detect when homed users are added/removed
- Create homed users
- Delete homed users
AccountsService cannot:
- Mutate homed users in any way (needs some support from homed)
- Change passwords (needs some support from homed)
- Create users w/o a password (needs some support from homed) - the users we create have a password set to "" (a blank string)
Work done as part of my efforts in integrating GNOME w/ homed. See here to better track my progress
Closes #89