Add sysctlbyname system call
Previously userspace would issue one syscall to resolve the sysctl and then
another one to actually use it. Do it all in one trip.
Fallback is provided in case newer libc happens to be running on an older
Submitted by: Pawel Biernacki
Reported by: kib, brooks
Differential Revision: https://reviews.freebsd.org/D17282