Sets the file position indicator for the file referenced by zp to offset bytes into the file stream. Equivalent to calling (in C) gzseek( zp, offset, SEEK_SET ).
If the file is opened for reading, this function is emulated but can be extremely slow. If the file is opened for writing, only forward seeks are supported; gzseek then compresses a sequence of zeroes up to the new starting position.
Upon success, returns 0; otherwise, returns -1. Note that seeking past EOF is not considered an error.