security/softether5: backport some patches from upstream
- Fix DNS resolution when no IPv6 address is configured on any interface [1]
- Disable MTU changes to avoid frequent interface up/down [2]
[1] https://github.com/SoftEtherVPN/SoftEtherVPN/pull/1510
[2] https://github.com/SoftEtherVPN/SoftEtherVPN/issues/1677
Obtained from: https://github.com/SoftEtherVPN/SoftEtherVPN/pull/1510
PR: 267178