[libstdc++] Partial implementation of DR 60.

[Benjamin Kosnik]

>DR 60 makes changes to which functions affect, or do not affect, subsequent
>calls to gcount().  Easily implemented as below.  I'd actually noticed
>these back when doxygenating the I/O library, but never checked them in.  (?)
>
>DR 60 also changes other things, not done here.

Ouch!!!! I had just changed some of this behavior when the testsuites
got re-packaged. I was sure that I had added tests for some of this
stuff. Perhaps not, if you are changing this behavior as well and not
noticing anything? Is there any chance I could get you to add some
simple tests for the functions you've modified? They don't have to be
anything more than one liners with a call to gcount. 

At least for basic_istream/[putback,seekg,tellg].

Also, both seekg's should have  _M_gcount = 0 removed.

>Should this go in 3.3?  *shrug*

I think you might as well put this in both places (you can keep the test
cases only in mainline if you'd prefer, since merging them can be a
pain.)

thanks,
-benjamin