"SHOW TABLE STATUS very slow w/large number of tables"
Replaced old algorithm which were used in my_dir() and stored
all information about directory entries in one chunk of memory
with new one which stores file names and MY_STAT structures in
separate memroot, so now we don't need to copy this data during
reallocation of dir_entry array.


include/my_dir.h:
  Changed mystat member of FILEINFO structure to pointer since 
  this prevents unneeded memory allocation and initialization.
  Added comment about new hidden members of MY_DIR structure.
mysys/my_lib.c:
  Replaced old algorithm in my_dir() which stored all information
  about directory entries in one chunk of memory with new one 
  which stores file names and MY_STAT structures in separate
  memroot. Now we don't copy this data during reallocation of 
  array with FILEINFO structures.
  Also tuned sizes of memory chunks during first-other
  reallocations (we suppose that we either have < 100 files 
  in the directory or > 1000 of them).
sql/sql_show.cc:
  Updated only place in code where mystat member
  of FILEINFO structure is used.