Document DNS client use and configuration

[jovatn]

This issue relates to #782 and should probably be a sub-issue of that.

DNS resolver can be configured statically (ietf-system) or received via DHCP client (infix-dhcp-client). Status can be viewed with "show dns".

• Examples should be added to doc/system.md, and
• possibly the behavior when receiving dns resolver settings from different sources (static, DHCP) should be explained
• (optional) relation to mDNS client (well, see Update discovery.md with infix.local mDNS alias #738)
• (optional) ... and possibly IPv6 vs IPv4 lookups.

Use of NTP client/server configuration is useful in example, as this is a case where DNS resolution is clearly needed.

admin@infix-00-00-00:/config/system/> help dns-resolver 
  search                            An ordered list of domains to search when resolving
  server                            List of the DNS servers that the resolver should query.
  options                           Resolver options.  The set of available options has been
admin@infix-00-00-00:/config/system/> 

admin@infix-00-00-00:/config/system/> edit ntp server ntpexample
admin@infix-00-00-00:/config/system/ntp/server/ntpexample/> set udp address ntp.pool.org
admin@infix-00-00-00:/config/system/ntp/server/ntpexample/>