Add --detach. Add multi-pass flag parsing that lets us get things in the right order.

adsbus/exec.h

@@ -2,5 +2,7 @@
 
 struct flow;
 
+void exec_opts_add(void);
+void exec_init(void);
 void exec_cleanup(void);
 void exec_new(const char *, struct flow *, void *);