tools: ynl: submsg: reverse parse / error reporting

Reverse parsing lets YNL convert bad and missing attr pointers
from extack into a string like "missing attribute nest1.nest2.attr_name".
It's a feature that's unique to YNL C AFAIU (even the Python YNL
can't do nested reverse parsing). Add support for reverse-parsing
of sub-messages.

To simplify the logic and the code annotate the type policies
with extra metadata. Mark the selectors and the messages with
the information we need. We assume that key / selector always
precedes the sub-message while parsing (and also if there are
multiple sub-messages like in rt-link they are interleaved
selector 1 ... submsg 1 ... selector 2 .. submsg 2, not
selector 1 ... selector 2 ... submsg 1 ... submsg 2).

The rt-link sample in a subsequent changes shows reverse parsing
of sub-messages in action.

Reviewed-by: Donald Hunter <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

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