Address some other compilation warnings.

interface.c

@@ -153,7 +153,7 @@ check_interface_ipv4(struct interface *ifp)
     return 0;
 }
 
-int
+static int
 check_interface_channel(struct interface *ifp)
 {
     int channel = IF_CONF(ifp, channel);
@@ -401,7 +401,8 @@ void
 check_interfaces(void)
 {
     struct interface *ifp;
-    int rc, ifindex, ifindex_changed = 0;
+    int rc, ifindex_changed = 0;
+    unsigned int ifindex;
 
     FOR_ALL_INTERFACES(ifp) {
         ifindex = if_nametoindex(ifp->name);

kernel.c

@@ -91,7 +91,7 @@ read_random_bytes(void *buf, size_t len)
 	rc = -1;
     } else {
         rc = read(fd, buf, len);
-        if(rc < len)
+        if(rc < 0 || (unsigned)rc < len)
             rc = -1;
         close(fd);
     }

kernel_socket.c

@@ -153,7 +153,7 @@ static int old_accept_redirects = -1;
 static int ifindex_lo = -1;
 static int seq;
 
-int
+static int
 mask2len(const struct in6_addr *addr)
 {
     int i = 0, j;
@@ -180,7 +180,7 @@ mask2len(const struct in6_addr *addr)
     return i;
 }
 
-void
+static void
 plen2mask(int n, struct in6_addr *dest)
 {
     unsigned char *p;

local.c

@@ -112,7 +112,7 @@ local_notify_self()
     return;
 }
 
-static char *
+static const char *
 local_kind(int kind)
 {
     switch(kind) {

message.c

@@ -1089,7 +1089,7 @@ buffer_update(struct interface *ifp,
     ifp->num_buffered_updates++;
 }
 
-void
+static void
 buffer_update_callback(struct babel_route *route, void *closure)
 {
     buffer_update((struct interface*)closure,