kernel: Drop non-standard GNU inline assignment syntax

* We needed this previously due to our gcc2 compiled kernel.
* Now that our kernel is always latest gcc, we can move to the
c++20 syntax for inline assignment.
* Improves compatibility with clang, less GNU-specific stuff

Change-Id: Ib7272a0a52554a31e9a0e788fd3f031db9049795
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5898
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

acpi_battery: use user_strlcpy in acpi_battery_read().

* also check for user addresses in acpi_battery_control().

acpi_button: use user_memcpy to write the user buffer.

* fix warning on x86_64.

acpi_button: Fix several issues with multiple devices.
- On some hardware, both the fixed function FADT as well as the
device-based power interfaces are present. In such a case we
would fail to publish one or the other, depending on which was
enumerated first, since we'd always attempt to publish the same
name regardless. Now we differentiate the device name for fixed vs
device.
- Only enable fixed function for actual fixed devices.
- Improve tracing.

acpi_button: fix build with gcc 4.9

power_daemon: use wait_for_objects() instead of polling.

* acpi_button and acpi_lid support select/deselect hooks.
* power_daemon now uses a thread waiting for lid or power button events.
* a power button event is still hardcoded to a shutdown.
* a lid event only displays a message.

acpi_button: improved traces

acpi_button: switch to fixed event and notify handlers.

* install a fixed event or notify handler depending on the device type.
* on handling, a non zero status is available for read.
* fixed #10154.

acpi_button: cleanup

acpi_button: migrated to the new driver API.

* HaikuImage: SYSTEM_ADD_ONS_DRIVERS_POWER is now used for new drivers.

acpi_button: Fix several issues with multiple devices.
- On some hardware, both the fixed function FADT as well as the
device-based power interfaces are present. In such a case we
would fail to publish one or the other, depending on which was
enumerated first, since we'd always attempt to publish the same
name regardless. Now we differentiate the device name for fixed vs
device.
- Only enable fixed function for actual fixed devices.
- Improve tracing.

acpi_button: fix build with gcc 4.9

power_daemon: use wait_for_objects() instead of polling.

* acpi_button and acpi_lid support select/deselect hooks.
* power_daemon now uses a thread waiting for lid or power button events.
* a power button event is still hardcoded to a shutdown.
* a lid event only displays a message.

acpi_button: improved traces

acpi_button: switch to fixed event and notify handlers.

* install a fixed event or notify handler depending on the device type.
* on handling, a non zero status is available for read.
* fixed #10154.

acpi_button: cleanup

acpi_button: migrated to the new driver API.

* HaikuImage: SYSTEM_ADD_ONS_DRIVERS_POWER is now used for new drivers.