Support historical hotplug

[lu-zero]

Linux may execute the binary set in /proc/sys/kernel/hotplug.

To support it we have to:

• Make sure being run by the kernel works as intended regarding stdio file descriptors
• Make sure the information received can be parsed by kobject-uevent and fed to the normal path.

[lu-zero]

Supporting it requires adding an external locking mechanism and overall it is a bad idea to use this mode, we could drop the feature.