Replace bcache-register with c, use builtin kmod

In a pure systemd pre-boot environment, register-bcache can't be shell
code as there is no shell available. Switch to loading the module with
udev's builtin kmod support and introduce a small binary for registering
devices.

Gabriel: Changed errors to be verbose
bcache-register: use register, not register_quiet

register_quiet doesn't report errors correctly, the following kernel
patch needs to be applied:  https://lkml.org/lkml/2013/8/26/549

See http://thread.gmane.org/gmane.linux.kernel.bcache.devel/2594
for how this comes up in practice.

Reverts 8327108eeaf3e0491b17d803da164c0827aae622