MDEV-20279 Increase Aria index length limit

Limit increased from 1000 to 2000.

Avoiding stack overflow by only storing keys and pages on the stack in
recursive functions if there is plenty of space on it.

Other things:
- Use less stack space for b-tree operations as we now only allocate as
  much space as needed instead of always allocating HA_MAX_KEY_LENGTH.
- Replaced most usage of my_safe_alloca() in Aria with the stack_alloc
  interface.
- Moved my_setstacksize() to mysys/my_pthread.c

include/myisam.h

@@ -309,6 +309,8 @@ extern int mi_make_backup_of_index(struct st_myisam_info *info,
 #define   MYISAMCHK_VERIFY 2  /* Verify, run repair if failure */
 
 typedef uint mi_bit_type;
+typedef struct st_sort_key_blocks SORT_KEY_BLOCKS;
+typedef struct st_sort_ftbuf SORT_FT_BUF;
 
 typedef struct st_mi_bit_buff
 {                                       /* Used for packing of record */