This struct is used for addition not only for multiplication, so
remove the word `mul`, and make the member names more descriptive.

Previously, warned only in `new` and `map`, but not `for` and
`string`.

https://hackerone.com/reports/3437743

At least, `string` in `io_buffer_set_string` can be different from
`argv[0]` after `rb_str_to_str` call.  The other cases may not be
necessary.

If we don't have uint128, then rb_int128_to_numeric emits a strict
aliasing warning:

    numeric.c:3641:39: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
    3641 |         return rb_uint128_to_numeric(*(rb_uint128_t*)&n);
         |                                       ^~~~~~~~~~~~~~~~~

The following warnings are emitted. We can use type punning to prevent
strict aliasing violations.

    io_buffer.c:1935:23: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
    1935 |     rb_uint128_t u = *(rb_uint128_t*)&x;
         |                       ^~~~~~~~~~~~~~~~~
    io_buffer.c:1937:13: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
    1937 |     return *(rb_int128_t*)&swapped;
         |

The method incorrectly ignored its first argument and treated the second argument as offset and the third as count.
This change makes the first argument be treated as offset and the second as count.

Also fix incorrect block parameter in comments.

- Buffer's size did not account for offset when mapping the file, leading to possible crashes.
- Size and offset were not checked properly, leading to many situations raising EINVAL errors with generic messages.
- Documentation was wrong.