scripts: Address shellcheck issues

[crawfxrd]

Report issues by shell files with:

git ls-files '*.sh' | xargs shellcheck --exclude=SC2162

Address the following:

• SC1087: Use braces when expanding arrays
• SC1091: Not following
• SC2004: $/${} is unnecessary on arithmetic variables
• SC2024: sudo doesn't affect redirects
• SC2034: foo appears unused. Verify it or export it
• SC2086: Double quote to prevent globbing and word splitting
• SC2087: Quote EOF
• SC2115: Use "${var:?}" to ensure this never expands to /*
• SC2148: Add a shebang

Addresses (at least partially) some POSIX/dash issues:

• SC2113: function keyword is non-standard
• SC3010: In POSIX sh, [[ ]] is undefined
• SC3014: In POSIX sh, == in place of = is undefined
• SC3020: In POSIX sh, &> is undefined
• SC3046: In POSIX sh, source in place of . is undefined

Does not address:

• SC2162: read without -r will mangle backslashes
• Any other POSIX/dash-specific issues