New review comment by tornaria on void-packages repository

https://github.com/void-linux/void-packages/pull/34757#discussion_r778486761

Comment:
The default is "none" which means there's no group. The only change with this seems to be in the file `/usr/lib/udev/rules.d/69-libmtp.rules`:
```diff
--- 69-libmtp.rules.old	2020-11-05 23:30:09.000000000 -0300
+++ 69-libmtp.rules.new	2021-12-29 18:26:43.895937081 -0300
@@ -9,7 +9,7 @@
 LABEL="libmtp_usb_rules"
 
 # If we have a hwdb entry for this device, act immediately!
-ENV{ID_MTP_DEVICE}=="1", SYMLINK+="libmtp-%k", GOTO="libmtp_rules_end"
+ENV{ID_MTP_DEVICE}=="1", SYMLINK+="libmtp-%k", GROUP="_libmtp", GOTO="libmtp_rules_end"
 
 # Fall back to probing.
 # Some sensitive devices we surely don't wanna probe
@@ -36,6 +36,6 @@
 ENV{ID_USB_INTERFACES}=="*:0701??:*", GOTO="libmtp_rules_end"
 
 # Autoprobe vendor-specific, communication and PTP devices
-ENV{ID_MTP_DEVICE}!="1", ENV{MTP_NO_PROBE}!="1", ENV{COLOR_MEASUREMENT_DEVICE}!="1", ENV{ID_GPHOTO}!="1", ENV{libsane_matched}!="yes", ATTR{bDeviceClass}=="00|02|06|ef|ff", PROGRAM="/usr/lib/udev/mtp-probe /sys$env{DEVPATH} $attr{busnum} $attr{devnum}", RESULT=="1", SYMLINK+="libmtp-%k", ENV{ID_MTP_DEVICE}="1", ENV{ID_MEDIA_PLAYER}="1"
+ENV{ID_MTP_DEVICE}!="1", ENV{MTP_NO_PROBE}!="1", ENV{COLOR_MEASUREMENT_DEVICE}!="1", ENV{ID_GPHOTO}!="1", ENV{libsane_matched}!="yes", ATTR{bDeviceClass}=="00|02|06|ef|ff", PROGRAM="/usr/lib/udev/mtp-probe /sys$env{DEVPATH} $attr{busnum} $attr{devnum}", RESULT=="1", SYMLINK+="libmtp-%k", ENV{ID_MTP_DEVICE}="1", ENV{ID_MEDIA_PLAYER}="1", GROUP="_libmtp"
 
 LABEL="libmtp_rules_end"
```

The net effect is that the device node that is created in `/dev/bus/usb/...` has its group set to `_libmtp`. These files are created with mode 664 in any case, but with group `root`.