cmd/dist: fix superfluous and confusing "binaries ... to be copied or moved" message

Also, to aid debugging cmd/dist, make make.bat support --dist-tool flag.

Fixes #3100.

R=alex.brainman
CC=golang-dev
https://golang.org/cl/6637061

src/cmd/dist/unix.c

@@ -727,5 +727,12 @@ xstrrchr(char *p, int c)
 	return strrchr(p, c);
 }
 
+// xsamefile returns whether f1 and f2 are the same file (or dir)
+int
+xsamefile(char *f1, char *f2)
+{
+	return streq(f1, f2); // suffice for now
+}
+
 #endif // PLAN9
 #endif // __WINDOWS__