PPP, from PPPD upstream, separate IPv6 handling for sifup/sifdown

The current code is buggy regarding handling of link state when using
both IPCP and IPv6CP: if IPv6CP has been set up and if during IPCP
negociation, ipcp_up() fails, it will incorrectly take the interface
down. The simple solution here is to change the platform code to do the
same as on Solaris: separate IPv6CP up/down state handling with sif6up()
and sif6down(), so that we really know when the interface is allowed to
go down.

(Based from pppd commit b04d2dc6df5c6b5650fea44250d58757ee3dac4a)

src/include/netif/ppp/ppp.h

@@ -379,6 +379,11 @@ struct ppp_pcb_s {
 
   /* flags */
   unsigned int if_up                   :1; /* True when the interface is up. */
+#if PPP_IPV6_SUPPORT
+  unsigned int if6_up                  :1; /* True when the IPv6 interface is up. */
+#else
+  unsigned int                         :1; /* 1 bit of padding */
+#endif /* PPP_IPV6_SUPPORT */
   unsigned int pcomp                   :1; /* Does peer accept protocol compression? */
   unsigned int accomp                  :1; /* Does peer accept addr/ctl compression? */
   unsigned int proxy_arp_set           :1; /* Have created proxy arp entry */
@@ -396,7 +401,7 @@ struct ppp_pcb_s {
 #else
   unsigned int                         :1; /* 1 bit of padding */
 #endif /* PPPOS_SUPPORT && VJ_SUPPORT */
-  unsigned int                         :6; /* 5 bits of padding to round out to 16 bits */
+  unsigned int                         :5; /* 5 bits of padding to round out to 16 bits */
 
 #if PPPOS_SUPPORT
 /* FIXME: there is probably one superfluous */

src/include/netif/ppp/ppp_impl.h

@@ -406,6 +406,11 @@ int cdns(ppp_pcb *pcb, u32_t ns1, u32_t ns2);
 int sifup(ppp_pcb *pcb);
 int sifdown (ppp_pcb *pcb);
 
+#if PPP_IPV6_SUPPORT
+int sif6up(ppp_pcb *pcb);
+int sif6down (ppp_pcb *pcb);
+#endif /* PPP_IPV6_SUPPORT */
+
 int sifnpmode(ppp_pcb *pcb, int proto, enum NPmode mode);
 
 void netif_set_mtu(ppp_pcb *pcb, int mtu);