Only clear variables after making them local

Make the config module to only clear the public names for the
configuration variables after having declared those local.

Otherwise, some shells (e.g. Mac OS X's 10.9 /bin/sh) treat the
variables as defined only by the mere fact of declaring them
local.  This completely broke default settings set by the caller
as they would be cleared after loading the file.
Add the bool module

This new module currently contains a single function, shtk_bool_check,
to convert a string to a boolean.
Add the cleanup module

This new module provides facilities to install termination handlers
that run on exit or on signal reception.