Add 'missing_ok' argument to build_attrmap_by_name

When it's given as true, return a 0 in the position of the missing
column rather than raising an error.

This is currently unused, but it allows us to reimplement column
permission checking in a subsequent commit.  It seems worth breaking
into a separate commit because it affects unrelated code.

Author: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CA+HiwqFfiai=qBxPDTjaio_ZcaqUKh+FC=prESrB8ogZgFNNNQ@mail.gmail.com
