Add helper functions for f_modify and f_process to condense code

Add helper functions for f_modify and f_process to condense code

These new functions, knote_modify() and knote_process(), implement
the logic that is common to most f_modify and f_process instances.

The code is inlined so as to not add yet another call frame on the
already towering stack of kqueue functions. Also, the _fn versions
allow direct calling of an event function when there is only one
filter type to handle.