Signed-off-by: David Bauer <mail@david-bauer.net>

This reverts commit 543eb178.

Restores the dns caching configuration that should be feasible, since we
now require around 8 MB of flash.

dnsmasq's caching is severly broken and does not handle all answer records
equally. In particular, its cached answers are missing DNSKEY and DS
records, breaking DNSSEC validation on clients.

Remove the cache for now. It may return if dnsmasq is fixed or we switch to
a different resolver.

Adding hints that the next_node name should contain a dot to avoid (mobile) browsers interpreting "nextnode" as searchphrase. Also setting it to an fqdn helps when clients use static dns servers.

gluon-core: dns-cache documentation polish
Now its clear that upstreams severs normally means "community gateways".
* added cacheentries to description
* made upstream server description more general
* description of the A and AAAA-records for next_node added
* mention of Cache not being allocated when not set in site.conf 
* dns-cache doku polish