Arcyd: refactor, extract new 'abdt_exhandlers'

Simplify abdi_processargs by extracting the exception handler factory
functions into their own module.

Test Plan:
$ ./precommit.sh
Arcyd: refactor, rename abdi_process(repo)args

Make the relationship of abdi_repoargs and abdi_processargs more obvious
by renaming abdi_processargs accordingly.

Rename 'run_once' to 'do' - there's no 'run_twice' or 'run_many' so it's
a bit redundant.

Test Plan:
$ ./precommit.sh