REGISTER command handler, userspace test program.

Wrote keymon_genl_register_cmd, that receives REGISTER command and prints PID
attribute value.

Wrote userspace program (src/daemon/keymond.c) successfully connects to
kernel-space generic netlink bus, find keymon family, and sends REGISTER
command. Yay!

Removed LINUX_VERSION_CODE from keymon_nla_policy because libnl doesn't support
NLA_S32 and it unnecessary complexity.