Merge branch 'ab/portable-more'

Portability fix. * ab/portable-more: tests: fix non-portable iconv invocation tests: fix non-portable "${var:-"str"}" construct tests: fix and add lint for non-portable grep --file tests: fix version-specific portability issue in Perl JSON tests: use shorter labels in chainlint.sed for AIX sed tests: fix comment syntax in chainlint.sed for AIX sed tests: fix and add lint for non-portable seq tests: fix and add lint for non-portable head -c N