listkeys: a few fixes

allocate each keys_return_type element.

handle REQ_TYPE_LISTKEYS at sock_scm_reader.

define list_keys_scm_complete prototype in frontend.h

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
listkeys: note we need to convert uid/gid

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>