ejabberd: start the daemon as ejabberd user

ejabberd.mk creates an ejabberd user but the init script was starting
the xmpp server as root user. This patch fixes it by invoking
ejabberctl from a "su ejabberd -c" command.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>