Meta: Fix shellcheck warnings in various scripts

Warnings fixed:
 * SC2086: Double quote to prevent globbing and word splitting.
 * SC2006: Use $(...) notation instead of legacy backticked `...`
 * SC2039: In POSIX sh, echo flags are undefined
 * SC2209: Use var=$(command) to assign output (or quote to assign string)
 * SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails
 * SC2166: Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.
 * SC2034: i appears unused. Verify use (or export if used externally)
 * SC2046: Quote this to prevent word splitting.
 * SC2236: Use -z instead of ! -n.

There are still a lot of warnings in Kernel/run about:
 - SC2086: Double quote to prevent globbing and word splitting.

However, splitting on space is intentional in this case, and not trivial to
change. Therefore ignore the warning for now - but we should fix this in
the future.

Kernel/sync.sh

@@ -1,4 +1,6 @@
 #!/bin/sh
+set -e
+
 script_path=$(cd -P -- "$(dirname -- "$0")" && pwd -P)
 cd "$script_path"