[Patch] Change _M_convert_to_external to return bool

[Benjamin Kosnik]

>this is the less controversial part of my small project of
>turning _M_overflow to call _M_convert_to_external just one
>time (instead of once for buffer + once for overflow char).

this is in with some changes. Your line wrapping didn't resolve the
situation with an 80-char display so I left it out.

-benjamin