uref_attr: add uref_##group##_match_##attr

Add uref_##group##_match_##attr for UNSIGNED and
SMALL_UNSIGNED.
Add upipe_match_attr
upipe_genaux: allow arbitrary (u64) getattr
upipe_sws_thumbs: flush
upipe_sws_thumbs: remove uref_mgr

Duplicate incoming uref instead of allocating new one.
Add upipe_probe_uref, upipe_multicat_probe