cpu rendezvous API committed.

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    I have committed a generic MI cpu rendezvous API.  Basically this is an
    API that says 'force cpu X, Y, and Z into the kernel and make them wait
    until I tell them they can continue and, oh by the way, have them execute
    the specified function(s) when I tell them to as well'.

    There are some similar, harwired, APIs that exist in the i386 code, to
    deal with cross-cpu TLB shootdowns and things like that.  I will be
    moving those APIs to the new machine independant API soon (so I don't 
    have to implement a dozen API's in the AMD64 code!).

    Also, Alan Cox and Tor found some truely horrible race conditions
    with the SMP TLB shootdown code in FreeBSD-5, FreeBSD-4, and other
    operating systems, and that means DFly too, which I hope to address 
    with this new API.  Just about all the PMAP code and some of the VM 
    Page code is effected by the race but it happens so rarely compared 
    to software bugs that it has gone virtually unnoticed forever.

						-Matt