systemd: enable syslog.socket by default

systemd: enable syslog.socket by default

The syslog daemon behaviour in our systemd system should be like the following
1. If ForwardToSyslog is disabled, no daemon should be started.
2. If ForwardToSyslog is enabled, daemon should be started via socket
   activation. This is very important for OE based system. Because unlike
   other distros, OE allows serveral syslog packages installed into one
   image, using ALTERNATIVE mechanism to manage them. If the syslog
   service is started regardless of socket activation, things crash.
   For example, if there's 'WantedBy=multi-user.target' in the '[Install]'
   section of busybox-syslog.service file, then this service would be started
   anyway, even if syslog.service doesn't point to it. In this case,
   /sbin/syslogd in busybox-syslog.service might be from other syslog packages
   like sysklogd or rsyslog.

This patch is series of patches to implement the above behaviour. The
syslog.socket needs to be enabled by default so that other syslog service
could correctly be activated via socket activation at runtime.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>