src/configfile.[ch]: Implement the cf_get_default_interval() function.

src/configfile.[ch]: Implement the cf_get_default_interval() function.

This should be able to replace "interval_g" in a global (i.e. non-plugin)
context, such as the interval in which timeouts are being checked.

The default value (10 seconds) is also configurable at compile time using
a define.