Reintroduce the possibility to hexdump(3) PCD <=> PICC transmissions.

Not a hack this time:
  - Use the hexdump(3) function of the system if it exists;
  - Build the subpart of FreeBSD's libutil if not (contrib);
  - Do this only if configured --with-debug.

The Mifare Classic / Ultralight code will be eventually changed to also provide this functionality.

contrib/libutil/Makefile.am

@@ -0,0 +1,10 @@
+# $Id$
+
+if WITH_DEBUG
+if !HAS_LIBUTIL
+noinst_LTLIBRARIES = libutil.la
+noinst_HEADERS = libutil.h
+
+libutil_la_SOURCES = hexdump.c
+endif # !HAS_LIBUTIL
+endif # WITH_DEBUG