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This is useful for thing like Item_true and Item_false that we
allocated and initalize once and want to ensure that nothing can
change them

Main changes:
- Memory protection is achived by allocating memory with mmap() and
  protect it from write with mprotect()
- init_alloc_root(...,MY_ROOT_USE_MPROTECT) will create a
  memroot that one can later use with protect_root() to turn it
  read only or turn it back to read-write. All allocations to this
  memroot is done with mmap() to ensure page alligned allocations.
- alloc_root() code was rearranged to combine normal and valgrind code.
- init_alloc_root() now changes block size to be power of 2's, to get less
  memory fragmentation.
- Changed MEM_ROOT structure to make it smaller. Also renamed
  MEM_ROOT m_psi_key to psi_key.
- Moved MY_THREAD_SPECIFIC marker in MEM_ROOT from block size (old hack)
  to flags.
- Added global variable my_system_page_size. This is initialized at
  startup.