uretprobes: implemented, thus remove -ENOSYS

  1/6 and 6/6 patches are here to enclose return probes implementation
as well as prohibit uprobe_register() routine with no consumer set.

v3 changes: (the patch is introduced in v3)
  - remove 'TODO' as return probes implemented now

Signed-off-by: Anton Arapov <anton@redhat.com>