Add parameter to hardwire clicintattr.trig values

clicintattr.trig allows run-time configurability of edge vs level and active high vs low.

CLIC states:

> Some implementations may want to save these bits so only certain trigger types are supported. In this case, these bits become hard-wired to fixed values (WARL).

This makes a lot of sense, as this is something that doesn't change at run-time. Will need a new eloboration time parameter.