dynamic user/group manager

 systemd-userdbd is a system service that multiplexes user/group lookups to all
 local services that provide JSON user/group record definitions to the system.
 In addition it synthesizes JSON user/group records from classic UNIX/glibc NSS
 user/group records in order to provide full backwards compatibility. It may
 also pick up statically defined JSON user/group records from drop-in files.