pgsql: Clarify handling of special-case values in bootstrap catalog dat

Clarify handling of special-case values in bootstrap catalog data.

I (tgl) originally coded the special case for pg_proc.pronargs as
though it were a kind of default value. It seems better though to
treat computable columns as an independent concern: this makes the
code clearer, and probably a bit faster too since we needn't do
work inside the per-column loop.

Improve related comments, as well, in the expectation that there
might be more cases like this in future.

John Naylor, some additional comment-hacking by me

Discussion: https://postgr.es/m/CAJVSVGW-D7OobzU=dybVT2JqZAx-4X1yvBJdavBmqQL05Q6CLw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/45c6d75f8cd2399181063b4e0ec11c2664ebeda6

Modified Files
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src/backend/catalog/Catalog.pm | 24 ++++++++++++++++--------
src/include/catalog/pg_proc.dat | 3 +++
src/include/catalog/reformat_dat_file.pl | 18 +++++++++---------
3 files changed, 28 insertions(+), 17 deletions(-)