pppd: Place plugin-specific options after `plugin name`.

Before this patch, if we specify a plugin specific option through
`pppd_options` in /etc/config/network, e.g. `rp_pppoe_verbose 1`, pppd
would quit with the following error in log.

    Wed Oct  9 09:42:58 2013 daemon.notice netifd: GORG (1689): /usr/sbin/pppd: unrecognized option 'rp_pppoe_verbose'
    Wed Oct  9 09:42:58 2013 daemon.err pppd[1689]: unrecognized option 'rp_pppoe_verbose'
    Wed Oct  9 09:42:58 2013 daemon.notice netifd: GORG (1689): pppd version 2.4.5
    Wed Oct  9 09:42:58 2013 daemon.notice netifd: GORG (1689): Usage: /usr/sbin/pppd [ options ], where options are:

This is due to the requirement that function add_option() should be
called by the plugin_init() function first before pppd can parse those
options.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

SVN-Revision: 38911