nut: fix no permissions to use USB UPS, and more

nut: fix no permissions to use USB UPS, and more

When a USB UPS is first configured, the permissions on the device under
`/dev/bus/usb` have not yet been set to allow the nut user access. This
resulted in errors such as:

Fri Feb 13 23:39:01 2026 daemon.debug upsd[3504]: [D1] mainloop: UPS
[eco550ups] is not currently connected, trying to reconnect
Fri Feb 13 23:39:01 2026 daemon.debug upsd[3504]: [D1] mainloop: UPS
[eco550ups] is still not connected (FD -1)
Fri Feb 13 23:39:03 2026 daemon.debug upsd[3504]: [D1] mainloop: UPS
[eco550ups] is not currently connected, trying to reconnect
Fri Feb 13 23:39:03 2026 daemon.debug upsd[3504]: [D1] mainloop: UPS
[eco550ups] is still not connected (FD -1)

or

Fri Feb 13 23:38:44 2026 daemon.err usbhid-ups[3083]: No matching HID
UPS found
Fri Feb 13 23:38:49 2026 daemon.warn procd: failed adding instance
cgroup for nut-server: No error information
Fri Feb 13 23:38:49 2026 daemon.err usbhid-ups[3115]: libusb1: Could not
open any HID devices: insufficient permissions on everything
Fri Feb 13 23:38:49 2026 daemon.err usbhid-ups[3115]: No matching HID
UPS found
Fri Feb 13 23:38:54 2026 daemon.warn procd: failed adding instance
cgroup for nut-server: No error information

and upsd would enter a procd crashloop.

We fix that by looking in `sysfs` (under `/sys/devices`) to find the
correct USB device and set its ownership and permissions to allow acces
to the user the driver is running under.

Copilot complained about a few things

* nut-server.init had potential word-splitting issues in various spots.
* it also had some commands missing an argument
* improved documentation was required to clarify a dependency
* an incorrect sed could mangle names as well as remove the intended
  name

Additionally, while fixing those issues the author noticed that the case
of multiple UPS devices with the same vendorid:productid were not
correctly handled. A check of the serial number, if provided, was added
along with a fallback to allowing NUT communications with all UPS
devices with a given vendorid:productid, if no serial number was given.

Improve efficiency and decrease McCabe complexity of
ensure_usb_ups_access, while also fixing Copilot complaints.

$@ in case is a problem, and we only handle the first parameter in any
event, so change $@ to "$1"

Copilot caught a missing 2>&1 and we silence some shellcheck
false positives

Signed-off-by: Daniel F. Dickinson <redacted>