Fixed RT 938 having to do with SIOCGIFCONF on Linux. We now do a first

pass in which we pass a null pointer for the buffer, and we will get
back the length of the buffer we should allocate. This is because
if you pass in a buffer that is too small, Linux (unlike other OSes)
will not tell you that it is too small by returning a length greater
than the length you passed in.

includes/cf/linux.h

@@ -151,6 +151,7 @@ extern int h_errno;
 
 #define ALIAS_NAMES_PERMUTED
 #define SKIP_DUMMY_INTERFACES
+#define SIOCGIFCONF_NULL_BUF_GIVES_CORRECT_LEN
 
 #ifdef NEED_PRAND_CONF
 #ifndef HAVE_DEV_RANDOM