xids: Dump and restore tasks' pgid and sid

This is preriquisity for terminals handling and just a good
practice to save and restore everything we can :)

Not all combinations are supported. All the problems we still
have come from the inability to attach to group/session with
ID no tasks own as its PID.

This can be workarounded by fork()-ing this pid temporarily,
but we'd rather think in the direction of modifying the kernel
to give us direct syscall for this (oh my...)

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>