Add VRegistry::clearModules() to allow for runtime vmodule changes
For example, a signal handler (or some other runtime event) can
cause a loading of a "vmodule" specification into a string, and passed
to VRegistry::setModules().  Without a VRegistry::clearModules(), new
vmodule specs are appended onto existing verbose logs.  With it, eg.:
  void log_sighandler(void) {
    const char * newVmoduleSpec = ...
    ELPP->vRegistry()->clearModules();
    ELPP->vRegistry()->setModules(newVmoduleSpec);
  }
Can allow dynamic changing of verbosity levels of modules.