(struct malloc_state): Replace max_fast with flags fields.

(struct malloc_state): Replace max_fast with flags fields.
(global_max_fast): New variable.
(set_max_fast): Change to not require arena parameter and to modify
global_max_fast.  Change all callers.
(get_max_fast): New macro.  Use this instead of directly accessing
now removed max_fast arena member.
(have_fastchunks): Use flags instead of max_fast.
(clear_fastchunks): Likewise.
(set_fastchunks): Likewise.
(contiguous): Likewise.
(noncontiguous): Likewise.
(set_noncontiguous): Likewise.
(set_contiguous): Likewise.
(malloc_init_state): Explicitly set FASTCHUNKS_BIT.  Call set_max_fast
only for main arena.