[Patch] Tweak unbuffered overflow, provide unbuffered seek testcases

[Paolo Carlini]

Hi,

you will find attached the seekoff/pos testcases split into
buffered/unbuffered (and very slightly enlarged).

Unfortunately seekpos testing revealed that unbuffered
overflow has to be slightly more complicated than we hoped, as
this excerpt from seekpos/char/4-io.cc shows:

    c1 = fb_03.snextc();         //current in pointer +1
    VERIFY( c1 == 't' );
    c2 = fb_03.sputc('\n');      //test current out pointer

Before actually doing unbuffered sputc (overflow, that is), we
must rewind -1, since the last underflow (upon snextc) led to
a file descriptor one position after 't'. Note that the
same rewind action is needed in the buffered case.

A minor nit: a new unbuffered sputbackc of a couple of days ago
(which I erroneously tweaked from the buffered version) had to
be also patched to the correct version.

Tested x86-linux.

Ok with everyone?

Paolo.

P.S. I have not even attempted a major rewrote of overflow trying to
unify the buffered and unbuffered cases: this way I don't risk to
broke the much more important buffered case ;) Perhaps later...

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