Move self-isolation code into a separate file

passt/pasta contains a number of routines designed to isolate passt from
the rest of the system for security.  These are spread through util.c and
passt.c.  Move them together into a new isolation.c file.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

conf.c

@@ -40,6 +40,7 @@
 #include "tcp.h"
 #include "pasta.h"
 #include "lineread.h"
+#include "isolation.h"
 
 /**
  * get_bound_ports() - Get maps of ports with bound sockets