Handle configuration parameter enable/disable for telline

By default all tellines are enabled, we can disable it by
uci set asterisk.tellineX.enabled=0
When the corresponding telline is disabled all events in
brcm_process_event() are ignored (offhook, onhook etc),
no dialtone played and no call proceeded. For incoming
INVITE there is no ringing on the telline, we respond
with "403 Forbidden" when the telline is disabled.

Signed-off-by: Grzegorz Sluja <grzegorz.sluja@iopsys.eu>