Rename the kern.vm.kmem.size tunable to the more logical vm.kmem_size. To

Rename the kern.vm.kmem.size tunable to the more logical vm.kmem_size.  To
assure backward compatibility (conditional on !BURN_BRIDGES), look it up
by its old name first, and log a warning (but accept the setting) if it
was found.  If both the old and new name are defined, the new name takes
precedence.

Also export vm.kmem_size as a read-only sysctl variable; I find it hard to
tune a parameter when I don't know its default value, especially when that
default value is computed at boot time.