libutil/stdlog: use size_t not int #2478

Job	Run time
validate commits	0s
Generate build matrix	8s
python linting	31s
spelling	6s
bookworm - test-install	12m 11s
bookworm - 32 bit	13m 4s
noble - test-install	12m 44s
el9 - test-install	11m 58s
alpine - test-install	12m 8s
fedora40 - test-install	11m 44s
jammy - test-install	11m 54s
focal - py3.8	16m 56s
bookworm - gcc-12,content-s3,distcheck	19m 39s
fedora40 - clang-18	17m 3s
coverage	23m 13s
el8 - ascii	20m 17s
el8 - test-install	12m 55s
el8 - system,coverage	7m 19s
inception	22m 10s
flux-pmix check	12m 57s
flux-accounting check	4m 41s
macos build only	2m 22s
flux-pam check	4m 9s
flux-sched check	1h 21m 53s
Generate docker manifest	1s
	5h 32m 3s

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