Added support to MEM_ROOT for write protected memory

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.

sql/sql_class.cc

@@ -748,8 +748,7 @@ THD::THD(my_thread_id id, bool is_wsrep_applier)
     will be re-initialized in init_for_queries().
   */
   init_sql_alloc(key_memory_thd_main_mem_root,
-                 &main_mem_root, ALLOC_ROOT_MIN_BLOCK_SIZE, 0,
-                 MYF(MY_THREAD_SPECIFIC));
+                 &main_mem_root, 64, 0, MYF(MY_THREAD_SPECIFIC));
 
   /*
     Allocation of user variables for binary logging is always done with main