[Bugs] #2457 UNSP: cannot connect to the owning users DBus session bus

[Sugar Labs Bugs]

#2457: cannot connect to the owning users DBus session bus
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    Reporter:  sascha_silbe               |          Owner:  silbe                      
        Type:  defect                     |         Status:  new                        
    Priority:  Unspecified by Maintainer  |      Milestone:  Unspecified by Release Team
   Component:  Rainbow                    |        Version:  Git as of bugdate          
    Severity:  Major                      |       Keywords:                             
Status_field:  New                        |   Distribution:                             
   Seeta_dev:                             |  
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 IPC in Sugar happens over the DBus session bus. So in order for activities
 to work, we need to allow them to connect to the session bus.

 There are currently several issues. I'm putting them inside a single
 ticket so we can decide on a design.

 1. Rainbow uses {{{unshare(CLONE_NEWNET)}}} which (on 2.6.31) apparently
 disables abstract unix sockets as well.
 2. DBus allows only the owning user itself to connect by default, so we
 need to provide a custom config file.
 3. We need a group that all "Rainbow user accounts" (resp. the ones that
 should get DBus access) are in. Because DBus looks up the remote user
 account in the account database, this needs to be reflected in NSS, not
 just added during invocation ({{{setgroups(2)}}}). The group name must be
 known when {{{dbus-daemon}}} is started as it needs to be in the custom
 config file.

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Ticket URL: <http://bugs.sugarlabs.org/ticket/2457>
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