Fix #62004: SplFileObject: fgets after seek returns wrong line

As it is, `::seek(0)` sets the file pointer to the beginning of the
file, but `::seek($n)` where `$n > 0` sets the file poi

Fix #62004: SplFileObject: fgets after seek returns wrong line

As it is, `::seek(0)` sets the file pointer to the beginning of the
file, but `::seek($n)` where `$n > 0` sets the file pointer to the
beginning of the following line, having line `$n` already read into the
line buffer. This is pretty inconsistent; we fix it by always seeking
to the beginning of the line.

We also add a test case for the duplicate bug #46569.

Closes GH-6434.

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