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.

util.h

@@ -66,7 +66,8 @@ char *humansize(double);
 mode_t parsemode(const char *, mode_t, mode_t);
 void putword(const char *);
 extern int recurse_follow;
-void recurse(const char *, void (*)(const char *, int), int);
+extern int recurse_samedev;
+void recurse(const char *, void (*)(const char *, int, void *), int, void *);
 #undef strtonum
 long long strtonum(const char *, long long, long long, const char **);
 long long enstrtonum(int, const char *, long long, long long);