Replace SCH_*InputFileHandler() functions with more general
SCH_*FileHandler(), where events are specified as a new parameter and
which will later support other file events, e.g. file ready for ouput
and exception.
The file handlers have two new parameters: file descriptor and event.
This adds a support for receiving samples over unix domain socket.
It's a better alternative to the SHM refclock, the resolution is not
limited to microseconds and it doesn't require polling.
