xlat.c: handle NULL xlat in lookup routines as incremental search

xlat.c: handle NULL xlat in lookup routines as incremental search

This is going to be used later in IFLA_AF_SPEC decoding.

* xlat.c (xlookup): Add static variable pos, store xlat there when it is
non-NULL, use pos for lookup.
(xlat_search, xlat_idx): Add static variables pos and memb_left, store
xlat and nmemb there when xlat is non-NULL, use them for lookup.
(printxvals_ex): Store the last non-NULL xlat in static variable "last".
If the first xlat argument is NULL, use "last" instead.

Co-Authored-by: Dmitry V. Levin <ldv@altlinux.org>