daemon: Make sure that vlog is initialized when a process daemonizes.

daemon: Make sure that vlog is initialized when a process daemonizes.

If a process daemonizes itself, then it should be possible to control that
process's log levels with "ovs-appctl vlog/set" and related commands.  The
vlog_init() function registers those commands.  But vlog_init() doesn't
normally get called until the first log message is issued.  This can take a
while, especially for ovs-controller, where I first noticed the problem.

This commit fixes the problem by calling vlog_init() from
daemonize_start(), which always gets called as a process daemonizes.