Push kernel lock down to sysctl(2).

Push kernel lock down to sysctl(2).

Unlock few obvious immutable or read-only variables from "kern.*" and
"hw.*" paths. Keep the rest variables locked as before, include pages
wiring. Use new sysctl_vs{,un}lock() functions introduced for thar
purpose.

In kern.* path:

 - KERN_OSTYPE, KERN_OSRELEASE, KERN_OSVERSION, KERN_VERSION -
   immutable;
 - KERN_NUMVNODES - read-only access to integer;
 - KERN_MBSTAT - read-only access to per-CPU counters;

In hw.* path:

 - HW_MACHINE, HW_MODEL, HW_NCPUONLINE, HW_PHYSMEM, HW_VENDOR,
   HW_PRODUCT, HW_VERSION, HW_SERIALNO, HW_UUID, HW_PHYSMEM64 -
   immutable;
 - HW_USERMEM and HW_USERMEM64 - `physmem' is immutable, uvmexp.wired
   is mutable but integer; read-only access to localy stored difference
   between `physmem' and uvmexp.wired;
 - `hw_vars' - read-only access to integers; some of them like
   HW_BYTEORDER and HW_PAGESIZE are immutable;

ok bluhm kettenis