kernel - Implementat much deeper use of shared VM object locks

kernel - Implementat much deeper use of shared VM object locks

* Use a shared VM object lock on terminal (and likely highly shared)
  OBJT_VNODE objects.  For example, binaries in the system such as
  /bin/sh or /usr/bin/make.

  This greatly improves fork/exec and related VM faults on concurrently
  executing binaries.  Most commonly, parallel builds often exec
  hundreds of thousands of sh's and make's.

  +50% to +100% nominal improved performance under these conditions.
  +200% to +300% improved poudriere performance during the depend
  stage.

* Formalize the shared VM object lock with a new API function,
  vm_object_lock_maybe_shared(), which determines whether a VM
  object meets the requirements for obtaining a shared lock.

* Adjust the vm_fault*() APIs to track whether the VM object is
  locked shared or exclusive on entry.

* Clarify that OBJ_ONEMAPPING is only applicable to OBJT_DEFAULT
  and OBJT_SWAP objects.

* Heavy work on the exec path.  Somewhat lighter work on the exit
  path.  Tons more work could be done.