Avoid commiting configs inbetween to decrease number of flash writes.

Fixes #714.

gluon-announced and gluon-announce are merged to gluon-respondd.

Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.

gluon-autoupdater: unconditionally perform an update 5 minutes after boot when the whole PRIORITY delay has passed

The probability field is removed, updates are performed once per night between
04:00 and 05:00.

All announce.d scripts have been moved to /lib/gluon/announce/announce.d
The script /lib/gluon/announce/announce.lua will collect all information
and output json.

Closes #31

This also fixes the type of a few fields which were strings instead of numbers.

On nodes with autoupdate disabled the cmdline parameter '-f' has no
effect. But using the autoupdater for manual updates is quite handy.

This will free about 5 to 12 MB of RAM even on lightly used devices
yielding plenty of RAM for autoupdate to succeed.

This makes gluon-autoupdater depend on coreutils-sha512sum when it's not
included in busybox.

This way we have a kind of load balancing among the mirrors.

This adds a list of known models to the readme.

Instead of a single url this patch allows to have a list of urls. So in case
of network instability a router can still reach local mesh-cloud server to pull
a update from. In this stage the autoupdater simply tries every mirror until
one passes all tests.

This also removes the need for the cleanup function. Instead the cleanup
code is moved in place.

Move building blocks of the update into seperate functions.

The approach with awk's rand() wasn't really random between across
devices: When srand() was called without arguments, time() was used as
seed, which of course is the same on all devices when the script is
called via cron at HH:00:00.

This patch instead uses /dev/urandom as source of random (we don't need
cryptographically strong random numbers, so urandom is just fine) but
still uses awk for the comparison as busybox's ash cannot deal with
floats in $(())