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_array.h

@@ -122,8 +122,8 @@ public:
   Dynamic_array(MEM_ROOT *root, uint prealloc=16, uint increment=16)
   {
     void *init_buffer= alloc_root(root, sizeof(Elem) * prealloc);
-    init_dynamic_array2(root->m_psi_key, &array, sizeof(Elem), init_buffer,
-                           prealloc, increment, MYF(0));
+    init_dynamic_array2(root->psi_key, &array, sizeof(Elem), init_buffer,
+                        prealloc, increment, MYF(0));
   }
 
   void init(PSI_memory_key psi_key, uint prealloc=16, uint increment=16)