Move the initialisation/tuning of nmbclusters from param.c/machdep.c
into uipc_mbuf.c. This reduces three sets of identical tunable code to
one set, and puts the initialisation with the mbuf code proper.

Make NMBUFs tunable as well.

Move the nmbclusters sysctl here as well.

Move the initialisation of maxsockets from param.c to uipc_socket2.c,
next to its corresponding sysctl.

Use the new tunable macros for the kern.vm.kmem.size tunable (this should have
been in a separate commit, whoops).