[v3] more istream::ignore cleanups

[Gabriel Dos Reis]

Paolo Carlini <pcarlini@suse.de> writes:

| The problem is, on 32-bit machines, LFS enabled, your file can be
| bigger that 2G, thus more than 2G chars to ignore, in general, whereas
| streamsize, being basically a ptrdiff_t, is still limited to 32-bit:
| we are at risk of *signed* integer overflow, which is absolutely
| verboten!!! Still, we want to allow the user to keep on using ignore,
| in the general, unlimited form, also on such platforms...

I think it is written somewhere in the rules that max() morally
corresponds to unbounded ignore, unless the stream is corrupted or EOF
is reached.

| If you want, I can add comments...

That would be most welcome.  Thanks!

-- Gaby