Fix watchdog receive socket creation without IPv6.

Fix watchdog receive socket creation without IPv6.

When IPv6 network is not available, it was possible that watchdog
process won't start.  Previously wd_create_recv_socket() issued
elog(ERROR) if creation or handling IPv6 socket failed. Unfortunately
at the time when wd_create_recv_socket() is called, the exception
stack is not established, and elog happily converts ERROR to FATAL,
which causes exiting watchdog process, thus exiting pgpool process.

To fix this, the elog(ERROR) calls are changed to elog(LOG).

Reported-by: Bo Peng (pengbo@sraoss.co.jp)
Discussion: https://github.com/pgpool/pgpool2/issues/99
Backpatch-through: v4.6