libflux: update API to use size_t where appropriate #9989

Job	Run time
python linting	26s
validate commits	9s
Generate build matrix	6s
spelling	7s
bookworm - test-install	12m 5s
bookworm - 32 bit	12m 54s
noble - test-install	12m 31s
el9 - test-install	12m 42s
alpine - test-install	12m 16s
fedora40 - test-install	11m 50s
jammy - test-install	11m 52s
focal - py3.8	17m 0s
bookworm - gcc-12,content-s3,distcheck	19m 48s
fedora40 - clang-18	16m 38s
coverage	24m 0s
el8 - ascii	19m 37s
el8 - test-install	12m 59s
el8 - system,coverage	7m 48s
inception	21m 20s
flux-pam check	4m 23s
flux-pmix check	12m 49s
flux-accounting check	4m 48s
macos build only	2m 17s
flux-sched check	1h 21m 47s
Generate docker manifest	1s
	5h 32m 13s

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