--- seek(offset[, whence])

    եݥ󥿤 offset ưޤwhence 
    ͤ IO::SEEK_SET λˤϥեƬ顢
    IO::SEEK_CUR λˤϸߤΥեݥ󥿤顢
    IO::SEEK_END λˤEOF offset ХȤư
    whence ξάͤ IO::SEEK_SET Ǥ

    offset ֤ؤΰư 0 ֤ޤ
    Ԥ㳰 Errno::EXXX ȯޤ

