kernel - Remove PG_ZERO and zeroidle (page-zeroing) entirely

kernel - Remove PG_ZERO and zeroidle (page-zeroing) entirely

* Remove the PG_ZERO flag and remove all page-zeroing optimizations,
  entirely.  Aftering doing a substantial amount of testing, these
  optimizations, which existed all the way back to CSRG BSD, no longer
  provide any benefit on a modern system.

  - Pre-zeroing a page only takes 80ns on a modern cpu.  vm_fault overhead
    in general is ~at least 1 microscond.

  - Pre-zeroing a page leads to a cold-cache case on-use, forcing the fault
    source (e.g. a userland program) to actually get the data from main
    memory in its likely immediate use of the faulted page, reducing
    performance.

  - Zeroing the page at fault-time is actually more optimal because it does
    not require any reading of dynamic ram and leaves the cache hot.

  - Multiple synth and build tests show that active idle-time zeroing of
    pages actually reduces performance somewhat and incidental allocations
    of already-zerod pages (from page-table tear-downs) do not affect
    performance in any meaningful way.

* Remove bcopyi() and obbcopy() -> collapse into bcopy().  These other
  versions existed because bcopy() used to be specially-optimized and
  could not be used in all situations.  That is no longer true.

* Remove bcopy function pointer argument to m_devget().  It is no longer
  used.  This function existed to help support ancient drivers which might
  have needed a special memory copy to read and write mapped data.  It has
  long been supplanted by BUSDMA.