pgsql: Further code review for new integerset code.

Further code review for new integerset code.

Mostly cosmetic adjustments, but I added a more reliable method of
detecting whether an iteration is in progress.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f7ff0ae8428f9001eab244cb2535f40d29f218e9

Modified Files
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src/backend/lib/integerset.c | 190 +++++++++++----------
src/test/modules/test_integerset/README | 8 +-
src/test/modules/test_integerset/test_integerset.c | 15 +-
3 files changed, 111 insertions(+), 102 deletions(-)