bgpd: justify capabilities for TOS setting

To set the TOS bits on TCP connections, platforms that restrict
capabilities need the priv level to be raised before the sockopt
is set, and this requires the ZCAP_NET_ADMIN priv.

* bgp_main.c: update _caps_p to include ZCAP_NET_ADMIN
* bgp_network.c
  * bgp_connect(): request ZPRIVS_RAISE/ZPRIVS_LOWER
  * bgp_listener(): request ZPRIVS_RAISE earlier
diff --git a/bgpd/bgp_main.c b/bgpd/bgp_main.c
index 9d14683..4a38c15 100644
--- a/bgpd/bgp_main.c
+++ b/bgpd/bgp_main.c
@@ -119,6 +119,7 @@
 {
     ZCAP_BIND, 
     ZCAP_NET_RAW,
+    ZCAP_NET_ADMIN,
 };
 
 struct zebra_privs_t bgpd_privs =