Although the sysctls are marked with CTLFLAG_RD and the values will stay
immutable, current sysctl implementation stores value pointer in
void* type, which means that const qualifier is discarded anyway
and some newer compilers complaint about that.
We can't use de-const trick in sysctl implementation, because in that
case we could miss an opposite situation where a const value is used
with CTLFLAG_RW sysctl.
Complaint from: gcc 4.4, clang
MFC after: 2 weeks
b3f689911f