[LinuxPPS] Maintain pps-source from userland

[f.hiesinger]

 Hi Folks,
 is it possible, to register and maintain a pps-source from userland 
 (e.g. pps_register_source(...), pps_event(...))?

 If not, which tradeoffs are taken, when switching from 
 PPS-Clock-discipline (127.127.22.0) to SHM-Driver (127.127.28.1)? Any 
 loss in accuracy?

 BR,
 Frank