kernel - usched_dfly revamp (6), reimplement shared spinlocks & misc others

kernel - usched_dfly revamp (6), reimplement shared spinlocks & misc others

* Rename gd_spinlocks_wr to just gd_spinlocks.

* Reimplement shared spinlocks and optimize the shared spinlock path.
  Contended exclusive spinlocks are less optimal with this change.

* Use shared spinlocks for all file descriptor accesses.  This includes
  not only most IO calls like read() and write(), but also callbacks
  from kqueue to double-check the validity of a file descriptor.

* Use getnanouptime() instead of nanouptime() in kqueue_sleep() and
  kern_kevent(), removing a hardware I/O serialization (to read the HPET)
  from the critical path.

* These changes significantly reduce kernel spinlock contention when running
  postgres/pgbench benchmarks.