[v3] __pool_alloc multi lock

[Paolo Carlini]

B. Kosnik wrote:

>This removes the single _S_lock, and its export from the libstdc++
>shared library. It's been replaced with an array of mutexes.
>  
>
Cool. A minor nit (I think is the same of a few days ago)

>+     size_t __i = ((__bytes + (size_t)_S_align - 1) / (size_t)_S_align - 1);
>+     return *_S_mutexes[__i - 1];
>  
>
Benjamin, here, if __bytes < (size_t)_S_align + 1 (and > 0) then __i == 
0 and
_S_mutexes is indexed to -1: watch the parentheses!?!

Paolo.