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.

include/my_alloc.h

@@ -50,11 +50,12 @@ typedef struct st_mem_root
      first free block in queue test counter (if it exceed 
      MAX_BLOCK_USAGE_BEFORE_DROP block will be dropped in 'used' list)
   */
-  unsigned int first_block_usage;
+  unsigned short first_block_usage;
+  unsigned short flags;
 
   void (*error_handler)(void);
 
-  PSI_memory_key m_psi_key;
+  PSI_memory_key psi_key;
 } MEM_ROOT;
 
 #ifdef  __cplusplus