Moves the position indicator of the stream f. Unlike the standard fseek()
function, this only supports forward movements relative to the current
position and in read-only streams, so don't use negative offsets.
RETURN VALUE
Returns zero on success or a negative number on error, storing the error
code in errno.
Note that seeking is very slow when reading compressed files, and so
should be avoided unless you are sure that the file is not compressed.
