Storing initialization config files in etcdir

When initializing the module command in a shell session, initialization configuration files stored in the defined configuration directory are taken into account if present instead of the configuration files stored in the initialization script directory. When they are stored in the configuration directory, these configuration files are named initrc and modulespath instead of respectively modulerc and .modulespath.

This change gives the ability for package manager to move these configuration files from the /usr directory to /etc.

The location of the installation of those files can be controlled with configure option --with-initconf-in, which accepts etcdir and initdir values.