add error message for vkernel started with vm.vkernel_enable=0

[Matthew Dillon]

:I'd like to add something like the attached patch to the vkernel to
:let the user know that vm.vkernel_enable = 0.  I spent quite a while
:today chasing down why mmap() was failing before remembering that I
:needed to turn vm.vkernel_enable on.
:
:Joe

    That's fine, but adjust the sysctl to only read the variable, not
    set it also (pass NULL for the newp argument to sysctlbyname).

						-Matt