thread stage 5: Separate the inline functions out of sys/buf.h, creating

thread stage 5: Separate the inline functions out of sys/buf.h, creating
sys/buf2.h (A methodology that will continue as time passes).  This solves
inline vs struct ordering problems.

Do a major cleanup of the globaldata access methodology.  Create a
gcc-cacheable 'mycpu' macro & inline to access per-cpu data.  Atomicy is not
required because we will never change cpus out from under a thread, even if
it gets preempted by an interrupt thread, because we want to be able to
implement per-cpu caches that do not require locked bus cycles or special
instructions.