Introduce printflags_ex function

Introduce printflags_ex function

Add printflags_ex extension over printflags64 that, similar
to printxvals, takes a NULL-terminated sequence of xlat pointers.

* defs.h (printflags_ex): New prototype.
(printflags64): Turn into a thin wrapper around printflags_ex.
* netlink.c (decode_nlmsg_flags): Use printflags_ex.
* util.c (printflags_ex): New function.
(printflags64): Remove.
* xlat/netlink_get_flags.in: Remove entries listed
in xlat/netlink_flags.in file.
* xlat/netlink_new_flags.in: Likewise.