malloc: Count tcache entries downwards

malloc: Count tcache entries downwards

Currently tcache requires 2 global variable accesses to determine
whether a block can be added to the tcache.  Change the counts array
to 'num_slots' to indicate the number of entries that could be added.
If 'num_slots' reaches zero, no more blocks can be added.  If the entries
pointer is not NULL, at least one block is available for allocation.

Now each tcache bin can support a different maximum number of entries,
and they can be individually switched on or off (a zero initialized
num_slots+entry means the tcache bin is not available for free or malloc).

Reviewed-by: DJ Delorie <dj@redhat.com>