* Remove DUK_USE_USER_DECLARE which hasn't been necessary since
Duktape 2.0. Add release note for removal.
* Downgrade or remove FIXMEs in config-options.
* Remove config/other-defines YAML files; they were out of date and
didn't really have a good use case.
* Fix Ecmascript -> ECMAScript spelling in feature and config options.
Also a few instances in debugger and cmdline example.
* Rename prepare_sources.py to configure.py; the operation is similar to
autoconf ./configure so the association is useful.
* Rename make_dist.py to dist.py.
* Generate only one source set (combined or separate) for one run of
configure.py.
* Change dist.py to run configure.py three times to generate the default
source sets.
* Use an autodeleted temporary directory for preparing sources, which
removes some of the manual temporary file handling.