Audit du(1) and refactor recurse()

While auditing du(1) I realized that there's no way the over 100 lines
of procedures in du() would pass the audit.
Instead, I decided to rewrite this section using recurse() from libutil.
However, the issue was that you'd need some kind of payload to count
the number of bytes in the subdirectories and use them in the higher
hierarchies.
The solution is to add a "void *data" data pointer to each recurse-
function-prototype, which we might also be able to use in other
recurse-applications.
recurse() itself had to be augmented with a recurse_samedev-flag, which
basically prevents recurse from leaving the current device.

Now, let's take a closer look at the audit:
1) Removing the now unnecessary util-functions push, pop, xrealpath,
   rename print() to printpath(), localize some global variables.
2) Only pass the block count to nblks instead of the entire stat-
   pointer.
3) Fix estrtonum to use the minimum of LLONG_MAX and SIZE_MAX.
4) Use idiomatic argv+argc-loop
5) Report proper exit-status.

chgrp.c

@@ -14,7 +14,7 @@ static gid_t gid = -1;
 static int   ret = 0;
 
 static void
-chgrp(const char *path, int depth)
+chgrp(const char *path, int depth, void *data)
 {
 	char *chownf_name;
 	int (*chownf)(const char *, uid_t, gid_t);
@@ -31,7 +31,7 @@ chgrp(const char *path, int depth)
 		weprintf("%s %s:", chownf_name, path);
 		ret = 1;
 	} else if (Rflag) {
-		recurse(path, chgrp, depth);
+		recurse(path, chgrp, depth, NULL);
 	}
 }
 
@@ -80,7 +80,7 @@ main(int argc, char *argv[])
 			ret = 1;
 			continue;
 		}
-		chgrp(*argv, 0);
+		chgrp(*argv, 0, NULL);
 	}
 
 	return ret;