Implement userspace modprobe

[stapelberg]

See https://lore.kernel.org/linux-wireless/CANnVG6=a3etRagG+RaSEH-b4_nfzxpEKffQtuMWrttrbgjunZQ@mail.gmail.com/ for context: some kernel modules (notably brcmfmac) request loading other modules (brcmfmac-wcc).

For now, we load these modules explicitly from the gokrazy/wifi program. But, it would be better to provide the userspace modprobe program that our kernel is configured for.

To keep the binary size low, we could probably symlink /gokrazy/modprobe to /gokrazy/init and conditionally execute the loadModule function when invoked as modprobe.