[v3] Implement DR 804, other small tweaks to <system_error>

[Paolo Carlini]

Paolo Carlini wrote:
> Chris Fairles wrote:
>   
>> Are you also working on the CD changes? (posix -> generic, class enum
>> errc etc.). I have it almost done minus the testsuite tweaks. Just
>> wondering so work isn't duplicated.
>>   
>>     
> No, no, go ahead. Thanks!
>   
PS: do not add at the same time the system_error constructors taking a
const char*, in case let's take care of that in a future pass also
changing the other exception classes...

Paolo.