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.
See also gztell() and
gzrewind().