hotplug scripts are sourced not exec'd so #!/bin/sh /etc/rc.common
doesn't pull in the functions defined in /etc/rc.common thus since
'enabled' isn't defined the following sequence always fails:

enabled miniupnpd || exit 0

Unfortunately sourcing /etc/rc.common doesn't appear to work so come up
with some alternatives.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

Massive cleanup of the shell script. Many issues were fixed.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Import miniupnpd from routing repo and bump to 20180422.

Drop 102-ipv6-ext-port.patch as this looks upstreamed in the pinhole
code to me.
Consolidate all other patches & update with a view to sending upstream.

Add support for runtime IGDv1 mode switch (default to IGDv2)

(not extensively) Tested-on: ar71xx Archer C7 v2 in IGDv1 compatibility
mode.  A variety of devices/applications appear to be able to create
mappings.

Have an attempt at resolving https://github.com/openwrt-routing/packages/issues/286
TL;DR miniupnpd rules get processed before fw3 rules and thus can
override existing/intended redirects.  Ideally the miniupnpd rules would
be last in the relevant chains, unfortunately fw3 can sometimes use the
last rule as a REJECT.  Put miniupnpd rules as penultimate.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>