`NFTA_RULE_HANDLE` vs. `NFTA_RULE_POSITION` attribute type for "rule add" and "rule insert" operations

this issue involves some reverse engineering done on my system:
```
cco@firiel:~$ uname -a
Linux firiel 5.8.0-63-generic #71-Ubuntu SMP Tue Jul 13 15:59:12 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
cco@firiel:~$ lsb_release -a
LSB Version: core-11.1.0ubuntu2-noarch:printing-11.1.0ubuntu2-noarch:security-11.1.0ubuntu2-noarch
Distributor ID: Ubuntu
Description: Ubuntu 20.10
Release: 20.10
Codename: groovy
```
I ran into some problems when trying use the `nftables.Conn.AddRule()` and `nftables.Conn.InsertRule()` for adding or inserting new rules in a specified position.

I have checked with `strace` which are the systemcalls to which the `nft` tool translates "rule add/insert with handle" operations; like this for example:
```
strace -o /tmp/nft_dump nft insert rule ip filter MONITORING handle 382 ip saddr @whitelist counter
```
I have seen in the output that _only_ the attribute `NFTA_RULE_POSITION` (0x6) is used in netlink messages for these operations; it should be filled in with the actual _handle_ where the rule should be added or inserted. here is the relevant `sendmsg()` sycall from the strace output:

```
sendmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12,
msg_iov=[
{iov_base=
[
{
{len=20, type=NFNL_MSG_BATCH_BEGIN, flags=NLM_F_REQUEST, seq=0, pid=0},
{nfgen_family=AF_UNSPEC, version=NFNETLINK_V0, res_id=htons(2560)},
{
{len=188, type=NFNL_SUBSYS_NFTABLES<<8|NFT_MSG_NEWRULE, flags=NLM_F_REQUEST|NLM_F_CREATE, seq=1, pid=0},
{
nfgen_family=AF_INET, version=NFNETLINK_V0, res_id=htons(0),
[
{{nla_len=11, nla_type=NFNETLINK_V1}, "\x66\x69\x6c\x74\x65\x72\x00"},
{{nla_len=15, nla_type=0x2}, "\x4d\x4f\x4e\x49\x54\x4f\x52\x49\x4e\x47\x00"},
{{nla_len=12, nla_type=0x6}, "\x00\x00\x00\x00\x00\x00\x01\x84"},
{{nla_len=128, nla_type=NLA_F_NESTED|0x4}, "\x34\x00\x01\x80\x0c\x00\x01\x00\x70\x61\x79\x6c\x6f\x61\x64\x00\x24\x00\x02\x80\x08\x00\x01\x00\x00\x00\x00\x01\x08\x00\x02\x00"...}
]
},
{
{len=20, type=NFNL_MSG_BATCH_END, flags=NLM_F_REQUEST, seq=2, pid=0},
{nfgen_family=AF_UNSPEC, version=NFNETLINK_V0, res_id=htons(2560)}
}
], iov_len=228}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 228
```
as far as I can tell from my reverse engineering sessions, `NFTA_RULE_HANDLE` is NOT used when adding or inserting rules with specified handle through the `nft` command line.

now, one can still use the APIs `nftables.Conn.AddRule`, `nftables.Conn.InsertRule` by just setting the `Rule.Handle` to `0` and the actual handle in `Rule.Position`

maybe:
1. this behaviour should be documented somehow in `nftables`;
2. encoding of `Rule.Handle` should be removed from the insert/add APIs