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.
Merge pull request #62 from freifunk-gluon/clientcount

mesh-batman-adv-core: add clientcount statistics