pacify -Wzero-as-null-pointer-constant

Alexander Sosedkinrequested to merge
asosedkin/cligen:Wzero-as-null-pointer-constant into main

Replace 0 with NULL in long_opts to get rid of the -Wzero-as-null-pointer-constant warnings.

Passes make mypy flake8 codegen docgen-man docgen-texi, which is what was passing on main. Best reviewed commit-by-commit because of non-determinism in fixtures.

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