Rename the environment variables that are used by Modules to track
loaded environment state (variables whose name starts with `MODULES_LM`).
A `__` prefix is added to the name of these variables to indicate that
they are intended for internal use only.
Change license of mb, mlprof, modulecmd, mpub, mrel, mt, mtreview,
nglfar2ccov and playdemo scripts from GPLv3+ to GPLv2+.
Align this way all files from the Modules project under the GPLv2+
license.
Consent has been obtained from copyright holder to perform such move (I
hold the full copyright for these script files).
Related to #389
