Add history and config-struct to recurse

For loop detection, a history is mandatory. In the process of also
adding a flexible struct to recurse, the recurse-definition was moved
to fs.h.
The motivation behind the struct is to allow easy extensions to the
recurse-function without having to change the prototypes of all
functions in the process.
Adding flags is really simple as well now.

Using the recursor-struct, it's also easier to see which defaults
apply to a program (for instance, which type of follow, ...).

Another change was to add proper stat-lstat-usage in recurse. It
was wrong before.

libutil/rm.c

@@ -10,10 +10,10 @@ int rm_rflag = 0;
 int rm_status = 0;
 
 void
-rm(const char *path, int depth, void *data)
+rm(const char *path, void *data, struct recursor *r)
 {
 	if (rm_rflag)
-		recurse(path, rm, depth, NULL);
+		recurse(path, NULL, r);
 	if (remove(path) < 0) {
 		if (!rm_fflag)
 			weprintf("remove %s:", path);