Config option to require sensors be present in sysfs

[msbarth]

A problem has been determined on some devices where its possible that sensor(s) may not have associated sysfs file(s) created when the driver is bound and hwmon is started for the device. Currently there is no check between what sensors are presented in sysfs to what's provided in the hwmon config file for the device, essentially making sensors for all devices optional for hwmon to create on dbus.

A hwmon config option is needed to require what's given in the config file to also be present in sysfs. When the sysfs file for a sensor does not exist, hwmon should either log an error and continue OR hwmon should exit. This requires additional definition on how the config entry is defined.