fzsl: fixup default config

Find the system config rooted at sys.prefix and use ~/.config/fzslrc
instead of ~/.config/fzsl so the latter directory can be created for
caches if the user desires.
dev:  fixup dev environment

- add dev-install target to makefile which builds an sdist and then uses
  pip to install it in the virtual environment.
- dev-env.sh uses the virtual environment instead of source files.