Add new triggering framework.

Add new triggering framework.

The new triggers consist of a set of structs and an API to manipulate
them.  Both logic and analog triggers are supported, in an unlimited
number of stages.

A single struct sr_trigger containing its stages and triggers is then
added to the session.

In case of a driver where the hardware supports triggering, the struct
is then converted and used to arm the hardware trigger. Drivers without
hardware trigger support, such as fx2lafw or multimeter drivers, use it
as the basis for a software-based trigger implementation instead.