Refactor perl calls through a single method

I don't know the perl/C interface well enough to figure out the best way
to standardize the many variants that we use to call perl code.  Perhaps
we should standardize the error messages, and put less knobs on the
boilerplate macro.
Added support for calling perl subs with various return types
Added a perl hook for :away

Note: The sepbar no longer distinguishes between AWAY, A-AWAY, and
Z-AWAY.
Added a hook for wakeup/user input.

I wanted to add a hook that got called on all user-input, but any
non-trivial use of that hook incurred too much overhead (about 7 ms per
paragraph for entry into perl, and another 100 ms or so per paragraph
for execution of the hook; tests run on pasting lorem ipusm, on
linerva).

This hook is called at most once a second (approximately).
Added perl methods to register and unregister idle watchers.