/*
 * Returns the current line number that the stream is at. This number is based
 * on the newline separator being "\n"
 *
 *    reader = Bzip2::Reader.new Bzip2.compress("a\nb")
 *    reader.lineno     # => 0
 *    reader.readline   # => "a\n"
 *    reader.lineno     # => 1
 *    reader.readline   # => "b"
 *    reader.lineno     # => 2

 * @return [Integer] the current line number
 */
static VALUE bz_reader_lineno(VALUE obj) {