( resource zp, int offset )
Sets the file position indicator for the given file pointer to the
given offset byte 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.