Log: Fix broken syslog name

BIRD passed string from configuration to openlog(), which kept it
internally. After reconfiguration the old string was freed, therefore
openlog had invalid copy.

Thanks to Chris Caputo for the original patch.

lib/string.h

@@ -30,6 +30,15 @@ static inline char *xbasename(const char *str)
   return s ? s+1 : (char *) str;
 }
 
+static inline char *
+xstrdup(const char *c)
+{
+  size_t l = strlen(c) + 1;
+  char *z = xmalloc(l);
+  memcpy(z, c, l);
+  return z;
+}
+
 #define ROUTER_ID_64_LENGTH 23
 
 #endif