Move the mbuf memory limit calculations from init_param2() to

Move the mbuf memory limit calculations from init_param2() to
tunable_mbinit() where it is next to where it is used later.

Change the sysinit level of tunable_mbinit() from SI_SUB_TUNABLES
to SI_SUB_KMEM after the VM is running.  This allows to use better
methods to determine the effectively available physical and virtual
memory available to the kernel.

Update comments.

In a second step it can be merged into mbuf_init().