"system variable query_prealloc_size can be only global"


include/my_sys.h:
  Fix for bug #1948:
  - added reset_root_defaults function declaration
mysys/my_alloc.c:
  Fix for bug #1948:
  - implementation and description of reset_root_defaults
sql/set_var.cc:
  Fix for bug #1948:
  - fix_thd_mem_root and fix_trans_mem_root after update triggers
    implemented for variables query_prealloc_size, query_alloc_block_size,
    trans_prealloc_size, trans_alloc_block_size