Copter: Battery Failsafe for FS_OPTIONS: Continue if in Guided

## Feature request
Please add Battery Failsafe On to make active FS_OPTIONS: Continue if in Guided.

**Is your feature request related to a problem? Please describe.**
FS_OPTIONS: Continue if in Guided works only when FS_GCS_ENABLE = 1, but in some cases it requires to keep vehicle in the air at any cost, while it's used as a relay for example. Landing on Battery FS will prevent it from stack or crash in case all control was lost.

**Describe the solution you'd like**
I suggest to make FS_OPTIONS: Continue if in Guided mode active when any of these conditions are set:
1. FS_GCS_ENABLE = 1
2. BATT_FS_LOW_ACT <> 0
3. BATT_FS_CRT_ACT <> 0

**Platform**
[+] Copter

Start by tracing Copter's battery failsafe handling and how FS_OPTIONS is evaluated in Guided mode, then inspect the effects of FS_GCS_ENABLE, BATT_FS_LOW_ACT, and BATT_FS_CRT_ACT. Confirm the intended behavior for each listed condition and verify that battery failsafe actions still prevent landing when continuation is enabled.

Written by the indexing model from the issue text.