(dhcpd/unbound) don't try to handle empty section, closes https://github.com/opnsense/core/issues/1320

Optimized dhcp_clean_leases()

This is a pretty good option for anybody with a lack of a viable
local mirror, sparked by talks of a South-East Asia region mirror.

FreeBSD added OpenVPN 2.4, but we're not ready yet.

(proxy) fix subnet computation using netaddr.IPNetwork, closes https://github.com/opnsense/core/issues/1309

(configd) add netaddr.IPNetwork to template helpers, for https://github.com/opnsense/core/issues/1309

(ids) add support for inline configuration settings (subscription based url's for example), add basic auth support.

Example supported format:

<?xml version="1.0"?>
<ruleset>
    <location url="https://www.snort.org/rules/snortrules-snapshot-2990.tar.gz?oinkcode=%%snort.oinkcode%%" prefix="Snort"/>
    <files>
        <file description="blacklist" url="inline::rules/blacklist.rules">snort.blacklist.rules</file>
    </files>
    <properties>
        <property name="snort.oinkcode" default=""/>
    </properties>
</ruleset>

---
Registers the setting "snort.oinkcode" which is used to construct the download url.
This commit doesn't include definitions for new content, in case someone wants to create a definition file, it should be easy now :)