handle simple bashisms:

- [[ ]] -> [ ]
- == -> =
- source -> .
- redirect of fd 200 is error in mksh, use fd 9
- &> /dev/null -> > /dev/null 2>&1
- useless function keyword
- echo -e -> printf

still left bash shebang which did not validate with checkbashism, mostly
due 'type' being reported as bashism