Fix heartbeat_device treatment.

Fix heartbeat_device treatment.

While processing pgpool.conf, heartbeat_device was mistakenly treated
and the first device was ignored. For example:

heartbeat_device0 = 'eth0'

the configuration process disregarded 'eth0' and acted as if no device
was set. Another example:

heartbeat_device0 = 'eth0;eth1'

"eth0" was simply ignored.

Reviewed-by: Bo Peng <pengbo@sraoss.co.jp>
Backpatch-through: v4.2