examples/nfs-ls: recursion and summary support

this adds recusrion and summary support, fixes some compiler warnings,
fixes exit codes and avoids a possible sprintf buffer overflow.

Signed-off-by: Peter Lieven <pl@kamp.de>
libnfs_zdr_opaque: make valgrind happy

valgrind complains about unitialized memory

Signed-off-by: Peter Lieven <pl@kamp.de>