getentropy(3): Fallback to kern.arandom sysctl on older kernels

On older kernels, when userspace program disables SIGSYS, catch ENOSYS and
emulate getrandom(2) syscall with the kern.arandom sysctl (via existing
arc4_sysctl wrapper).

Special care is taken to faithfully emulate EFAULT on NULL pointers, because
sysctl(3) as used by kern.arandom ignores NULL oldp. (This was caught by
getentropy(3) ATF tests.)

Reported by: kib
Reviewed by: kib
Discussed with: delphij
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14785