Allow pasta to take a command to execute

When not given an existing PID or network namspace to attach to, pasta
spawns a shell.  Most commands which can spawn a shell in an altered
environment can also run other commands in that same environment, which can
be useful in automation.

Allow pasta to do the same thing; it can be given an arbitrary command to
run in the network and user namespace which pasta creates.  If neither a
command nor an existing PID or netns to attach to is given, continue to
spawn a default shell, as before.

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

pasta.h

@@ -6,7 +6,7 @@
 #ifndef PASTA_H
 #define PASTA_H
 
-void pasta_start_ns(struct ctx *c);
+void pasta_start_ns(struct ctx *c, int argc, char *argv[]);
 void pasta_ns_conf(struct ctx *c);
 void pasta_child_handler(int signal);
 int pasta_netns_quit_init(struct ctx *c);