quagga/ripd: Fix two bugs after received SIGHUP signal

There are two problems for ripd implementation after received
SIGHUP signal:
1). ripd didn't clean up ifp->connected list before reload
    configuration file which makes the same advertise packet
    being sent multiple times(depends on how many SIGHUP was recieved).
2). ripd reset ri->split_horizon flag to RIP_NO_SPLIT_HORIZON
    during restart which is different from the flag when ripd is
    firstly started up, leading to unnecessary route to be advertised.

[YOCTO #5266]

Signed-off-by: Xufeng Zhang <xufeng.zhang@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>