posix shell: use /bin/sh instead of bash

This uses the systems default shell:
- Ubuntu: dash
- Fedora: bash

Since bash is invoked via /bin/sh, it operates in POSIX mode:
  https://tiswww.case.edu/php/chet/bash/POSIX

- remove '# Ignore the expand_aliases command in zshell.'
  Not needed because the shell operates in POSIX mode
- [[ is bashism -> use [
- autostart_files=( $autostart_file_match )
  is not supported in dash, so use 'ls'
- shellcheck runs the dash flavor, since dash is a minimalistic shell.

Tested on dash & bash.

Tools/run-shellcheck.sh

@@ -32,6 +32,7 @@ shellcheck -x \
 	-e SC2148 \
 	-e SC2166 \
 	-e SC2039 \
+	--shell=dash \
 	$scripts
 ret=$?
 if [ $ret -ne 0 ]; then