luci-app-dnscrypt-proxy: small fixes & additions

* check/add for 'global' section to fix a possible issue with old
(non-default) dnscrypt-proxy config files
* a custom 'resolv-crypt.conf' can now be added in the new "Extra
options" section
* accept IPv4 and IPv6 address as local dnscrypt ip address

Signed-off-by: Dirk Brenken <dev@brenken.org>