Reintroduce the ``silent`` verbosity level and associated ``--silent``
and ``-s`` command-line switches, which were available on Modules v3.
Silent mode turns off error, warning and informational messages. The
``module`` command could be set silent by default at ``./configure``
time with ``--with-verbosity`` option. ``MODULES_VERBOSITY`` environment
variable supersedes this setting and it could be set with ``config``
module sub-command through the ``verbosity`` option.