[Patch] Avoid using _M_move_out_cur in stringbuf/seeks

[Benjamin Kosnik]

>this only a first preparatory step toward major changes to
>stringbuf and filebuf: elimination of _M_out_lim, simplification
>of _M_move_in_cur and _M_move_out_cur to pbump and gbump.

Right on.

>Anyway, the below is really simple minded: we cannot seek
>beyond _M_out_lim, therefore _M_move_out_cur boils down
>to simply updating _M_out_cur. Overall, the code becomes
>simpler and symmetric get area <-> put area.

Looks good.

-benjamin