winbind:varlink: Implement memberships enumeration

winbind:varlink: Implement memberships enumeration

$> varlink call -m unix:/run/systemd/userdb/org.samba.winbind/io.systemd.UserDatabase.GetMemberships "{\"service\":\"org.samba.winbind\"}"
{
  "groupName": "AFOREST+schema admins",
  "userName": "AFOREST+administrator"
}
{
  "groupName": "AFOREST+enterprise admins",
  "userName": "AFOREST+administrator"
}
{
  "groupName": "AFOREST+domain admins",
  "userName": "AFOREST+administrator"
}
{
  "groupName": "AFOREST+domain users",
  "userName": "AFOREST+administrator"
}
{
  "groupName": "AFOREST+domain users",
  "userName": "AFOREST+user1"
}
{
  "groupName": "AFOREST+domain users",
  "userName": "AFOREST+krbtgt"
}
{
  "groupName": "AFOREST+domain guests",
  "userName": "AFOREST+guest"
}
{
  "groupName": "AFOREST+group policy creator owners",
  "userName": "AFOREST+administrator"
}
{
  "groupName": "AFOREST+denied rodc password replication group",
  "userName": "AFOREST+krbtgt"
}

Signed-off-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>