security/openvpn: cherry-pick fixes from git repo

• 098edbb1 2020-05-20 | Switch assertion failure to returning false [Jeremy Evans]
• fc029714 2020-05-30 | pool: prevent IPv6 pools to be larger than 2^16 addresses [Antonio Quartulli]
• 38b46e6b 2020-02-20 | Persist management-query-remote and proxy prompts [Selva Nair]

MFH: 2020Q2 (blanket approval for stability fixes)