port-tell is not the same as ftell about gauche HOT 4 CLOSED

Thanks to catch this. Port-seek eventually calls lseek instead of fseek, for we don't use FILE* layer, but it's effectively the same for this issue. The problem is that port-seek tries to "optimize", that is, it avoids calling lseek if we "know" the current position. And the appending case we incorrectly assume we know the initial position. Will fix.

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Thanks. Btw, Gauche's manual points out the equivalence between port-tell and port-seek, it just happens to be confusing in this particular case.

I found a similar (but different) issue in chibi-scheme, already reported it.

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Oops, nope, the analysis above was wrong. We do call lseek(2) in the case of initial port-tell right after appending open, and lseek(2) returns zero in that case. It seems that we need explicit seek to the end to make lseek return a correct number.

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Fix pushed 946ac64

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