[Patch] Fix libstdc++/12232

[Paolo Carlini]

Hi,

this clean up (or variants of it ;) should be applied as soon as possible
in order not to block more substantive improvements to seekoff and seekpos.

It also allows nice simplifications (e.g., if open succeeds, either
ios_base::in & this->_M_mode is true or ios_base::out & this->_M_mode is
true; low level, lseek, fseek and such never had the equivalent of an
openmode parameter).

Tested x86-linux. Ok with everyone?

Paolo.

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