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>

isolation.h

@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later
+ * Copyright Red Hat
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ */
+
+#ifndef ISOLATION_H
+#define ISOLATION_H
+
+void drop_caps(void);
+void drop_root(uid_t uid, gid_t gid);
+int sandbox(struct ctx *c);
+void seccomp(const struct ctx *c);
+
+#endif /* ISOLATION_H */