sysutils/cpupdate: update to g20180320

Additional port changes:

• move binary from /bin/ to /sbin/ as this is sysadmin's utility;
• supply startup script for boot time updates started as early as possible, before configuration of network interfaces;
• make code respect PREFIX;
• patch source to kldload("cpuctl") automatically, if needed;
• fix some compiler warning;
• update pkg-descr to mention that CPU microcode updates bringing new features like IBRS require recent FreeBSD version for sake of CPUCTL_EVAL_CPU_FEATURES support.