apc_modbus: Fix outlet group command handling to be dynamic

Replace hardcoded outlet group commands with dynamic detection and
handling based on the `SOGRelayConfigSetting` register. This ensures
commands and variables are only exposed for outlet groups that are
actually present on the UPS.

Key changes:

- Add `APC_VF_UNAVAILABLE` flag to mark registers for absent outlet
  groups
- Add outlet group info table tracking presence and target bits for each
  group (`MOG`, `SOG0` to `SOG2`)
- Dynamically enumerate `outlet.group.N.id`,
  `outlet.group.N.designator` and `outlet.group.N.status` variables
  based on detected groups
- Replace static outlet command entries with
  `_apc_modbus_handle_outlet_cmd()` which parses `load.*`, `shutdown.*`,
  and `outlet.group.N.*` commands at runtime
- Global `load.*` and `shutdown.*` commands now target all available
  outlet groups instead of only the main outlet group (`MOG`)
- Add `outlet.group.N.*` commands (`load.off`, `load.on`, `load.cycle`,
  etc.) for each detected group
- Update `upsdrv_shutdown()` to use the unified command handler
- Add `OutletStatus_BF` bitfield definitions to header

This fixes issues where commands were exposed for non-existent outlet
groups and ensures load/shutdown operations affect all outlets on UPS
units with switched outlet groups.

Signed-off-by: Axel Gembe <axel@gembe.net>

Author: EchterAgo

NEWS.adoc

@@ -636,6 +636,11 @@ several `FSD` notifications into one executed action. [PR #3097]
  - `apc_modbus` driver updates:
    * Fixed register range in a logging string.
    * Add outlet group timers.
+   * Fixed outlet group command handling to dynamically detect available groups
+     (MOG, SOG0 - SOG2) in the device. Commands and variables are now only
+	 exposed for outlet groups actually present on the UPS. Global `load.*` and
+	 `shutdown.*` commands now correctly target all available outlet groups.
+	 [PR #3395]
 
 
 Release notes for NUT 2.8.4 - what's new since 2.8.3