dns/dnscrypt-proxy2: update 2.0.17 to 2.0.18

• official builds now support TLS 1.3;
• timeout for the initial connectivity check can be set from the cli;
• an 'Accept:' header is now always sent with GET queries;
• BOM is now ignored in configuration files;
• HTTP and HTTPS proxies are now supported for DoH servers.
  
  https://github.com/jedisct1/dnscrypt-proxy/blob/2.0.18/ChangeLog

Approved by: araujo (mentor)
Differential Revision: https://reviews.freebsd.org/D18005