bgpd: minor header/API clean up

Adds "const" on:
- peer_update_source_addr_set()
- peer_description_set()
Adds parameter names on:
- bgp_timers_set()
  (really confusing, this one, with 2 unexplained args of same type)
Adds new setter:
- peer_afc_set(), calling peer_activate/peer_deactivate.
  (intended for API consumers, matches peer->afc)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/bgpd/bgpd.c b/bgpd/bgpd.c
index e86665b..9521927 100644
--- a/bgpd/bgpd.c
+++ b/bgpd/bgpd.c
@@ -1179,6 +1179,15 @@
   return 0;
 }
 
+int
+peer_afc_set (struct peer *peer, afi_t afi, safi_t safi, int enable)
+{
+  if (enable)
+    return peer_activate (peer, afi, safi);
+  else
+    return peer_deactivate (peer, afi, safi);
+}
+
 static void
 peer_nsf_stop (struct peer *peer)
 {
@@ -2905,7 +2914,7 @@
 
 /* Neighbor description. */
 int
-peer_description_set (struct peer *peer, char *desc)
+peer_description_set (struct peer *peer, const char *desc)
 {
   if (peer->desc)
     XFREE (MTYPE_PEER_DESC, peer->desc);
@@ -2998,7 +3007,7 @@
 }
 
 int
-peer_update_source_addr_set (struct peer *peer, union sockunion *su)
+peer_update_source_addr_set (struct peer *peer, const union sockunion *su)
 {
   struct peer_group *group;
   struct listnode *node, *nnode;