isolation: Clarify various self-isolation steps

We have a number of steps of self-isolation scattered across our code.
Improve function names and add comments to make it clearer what the self
isolation model is, what the steps do, and why they happen at the points
they happen.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

isolation.h

@@ -7,9 +7,9 @@
 #ifndef ISOLATION_H
 #define ISOLATION_H
 
-void drop_caps(void);
+void isolate_initial(void);
 void isolate_user(uid_t uid, gid_t gid, bool use_userns, const char *userns);
-int sandbox(struct ctx *c);
-void seccomp(const struct ctx *c);
+int isolate_prefork(struct ctx *c);
+void isolate_postfork(const struct ctx *c);
 
 #endif /* ISOLATION_H */