libwebsockets: Only enable ipv6 if the build is ipv6 enabled

Without this, builds on AA will appear to work, with log messages of
"opening websockets listen socket on xxx" but won't actually open them,
as shown by netstat.
libwebsockets: add openssl/cyassl variants

Build two binaries, one for OpenSSL and one for CyaSSL.
CyaSSL version requires a patch, see
https://github.com/warmcat/libwebsockets/pull/161

According to comments on libwebsockets, cyassl needs to be recompiled to
actually _work_  This has only been compile tested.
libwebsockets: drop packaging for example apps

Example apps are useful when you're developing locally, but there's no
reason to build or offer to install 600k+ of test apps for OpenWrt
packages.

(Also, it's very complicated to build them against the right ssl layer
when you allow both to be built)
mosquitto: bump to 1.4-development branch

This is an unreleased preview image.

libwebsockets support is included and enabled by default.  You will need
to build the package yourself to disable this, it's not considered a
variant useful enough to have it's own binaries built.

Note that libwebsockets-openssl is chosen at present, as that's the ssl
library supported by mosquitto.

If you really don't want web sockets support, install the -nossl
variant, or rebuild mosquitto yourself, deselecting the websockes
support option.
mosquitto: Remove workarounds to skip building docs

mosquitto 1.4+ has direct config support for this, so no need for
overriding openwrt's default build rules.
mosquitto: allow building on older trees

mosquitto > 1.3 (1.4 dev branches and later) have support for elliptic
curve encryption, but AA and older openwrt branches don't have an ssl
library supporting that.  Make sure that EC is disabled on older builds.
mosquitto: overhaul uci conversion script

Support more options coming with mosquitto 1.4
Use more built in functions to make script simpler and easier to extend
mosquitto: libuuid now a dependency for auto client generation

Not required by mosquitto, but always provided by openwrt, so add it to
the dependencies list.