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

(cherry picked from commit a2224f647693714ea938138649f4c5d24627ae3e)