mesh-batman-adv-core: add clientcount statistics

This adds

  "client" { "total": <int>, "wifi": <int>" }

to statistics.d. "total" will be the number of clients connected.
"wifi" will be the number of clients connected over wifi. I.e. "total"
will always be equal to or greater than "wifi".

The node will not count itself.
Beschreibungstext umformuliert, damit die Option "Mesh-VPN" klarer wird.

Die Bezeichnung "Mesh-VPN" ist etwas irreführend. Man kann denken, dass hiermit das Meshing aktiviert wird, wobei es ja — im Gegenteil — um eine Nutzung des WAN geht.
gluon-mesh-vpn-fastd: delete all old backbone peers on update
gluon-setup-mode: cleanup and upgrade fix

Since switching to Barrier Breaker/procd, we'd not notice if we were
upgrading from a version before renaming the config file to gluon-setup-mode
as the upgrade scripts run after the preinit.

Fix this by checking later during setup mode init again and rebooting into the
regular run mode if we detect this case.
Remove gluon-ath9k-workaround
add overlay usage to statistics.d
overlay: change to normalized, fix error handling
overlay: rename to clarify value
adapt overlay_usage to match the comments from luebeck
calculate rootfs usage instead of overlayfs

in case a target does not use overlayfs
Merge pull request #65 from ohrensessel/overlayusage

add overlay usage to statistics.d
Merge pull request #62 from freifunk-gluon/clientcount

mesh-batman-adv-core: add clientcount statistics
gluon-announced: avoid spawning zombies
gluon-cron: Fix endless loop parsing invalid lines

Using the line
```
* * * * echo "foobar"
```
(notice the missing fifth time field) in a crontab causes gluon-cron
to enter an endless loop while parsing it, thus it won't even execute
the other, valid crontabs.

This is caused by the loop in [line 138] where `begin - min`
substracts the unsigned `min` from the signed `begin`. If now `begin`
is invalid, `strict_atoi` returns -1 and the loop starts at
`(-1)-1=MAX_INT` and runs while `i <= MAX_INT` which is always true.

The real culprit lies in [line 134] where exactly this case
`begin < min` is checked - but because of the signedness, this check doesn't
work as expected either.

The easiest solution is to make `min` a signed integer instead of an unsigned
one, as we do not require it to be very large and only pass the constants 0 or
1 to it.

To avoid other similar problems, this patch makes the input variable `n` a
signed integer as well.
Merge pull request #68 from FreifunkBremen/gluon-cron-fix

gluon-cron: Fix endless loop parsing invalid lines
gluon-luci-admin: restructure firmware upgrades
gluon-core: Remove VERSION_DIR hack

As the (removed) comment said, this was a one-time-hack that was obsoleted long ago.
mesh-vpn-fastd: enable from site.conf

This patch allows fastd's enabled flag's default value to be set from
site.conf.
Merge pull request #70 from jplitza/patch-2

gluon-core: Remove VERSION_DIR hack
Merge pull request #72 from freifunk-gluon/fastd-site-enable

mesh-vpn-fastd: enable from site.conf