mods-available and mods-enabled for v2.1.x

[Fajar A. Nugraha]

Hi Alan,

I've ported mods-available and mods-enabled to v2.1.x, but with a
different approach that what was implemented in master. It's currently
a patch to the debian directory which will only affect debian package.
Full commit: https://github.com/fajarnugraha/freeradius-server/commit/a4e2942

The idea is:
- exisisting debian/ubuntu with freeradius package installed can
upgrade to latest version and (if they choose to keep old
radiusd.conf) have everything continue to work just as it was before
(modules are loaded from modules/ directory)
- new users (or those chose to replace radiusd.conf with the one in
the package) will get 3.x behavior (modules loaded from mods-enabled/)
- mods-availabe is just a symlink to modules
- everything that's in mods-available (or to be accurate, everything
listed in DEFAULT_MODULES) is symlinked to mods-enabled

If you're interested in applying the change upstream (not just for
debian build), the alternate commit is here:
https://github.com/fajarnugraha/freeradius-server/commit/284198c

-- 
Fajar