pasta: By default, quit if filesystem-bound net namespace goes away

This should be convenient for users managing filesystem-bound network
namespaces: monitor the base directory of the namespace and exit if
the namespace given as PATH or NAME target is deleted. We can't add
an inotify watch directly on the namespace directory, that won't work
with nsfs.

Add an option to disable this behaviour, --no-netns-quit.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

pasta.h

@@ -6,3 +6,5 @@
 void pasta_start_ns(struct ctx *c);
 void pasta_ns_conf(struct ctx *c);
 void pasta_child_handler(int signal);
+int pasta_netns_quit_init(struct ctx *c);
+void pasta_netns_quit_handler(struct ctx *c, int inotify_fd);