MXS-2329 Allow the restriction of duration units

It's now possible to specify in the config parameter declaration
that the smallest allowed unit is seconds. For parameters whose
granularity is seconds, allowing to specify a duration in
milliseconds would open up a possibility for hard to detect errors.

include/maxscale/modinfo.h

@@ -109,6 +109,7 @@ enum mxs_module_param_options
     MXS_MODULE_OPT_PATH_F_OK   = (1 << 4),  /**< PATH: Path must exist */
     MXS_MODULE_OPT_PATH_CREAT  = (1 << 5),  /**< PATH: Create path if it doesn't exist */
     MXS_MODULE_OPT_ENUM_UNIQUE = (1 << 6),  /**< ENUM: Only one value can be defined */
+    MXS_MODULE_OPT_DURATION_S  = (1 << 7),  /**< DURATION: Cannot be specified in milliseconds */
 
     /**< Parameter is deprecated: Causes a warning to be logged if the parameter
      * is used but will not cause a configuration error. */