Replace bash with sh

[chregu]

Using /bin/sh instead of bash would be nicer. Due to some bashism in the current scripts, it's not that easy.
Also some scripts need to know the directory where the script is located, which seems to be hard to figure out in a pure POSIX way, see https://stackoverflow.com/questions/29832037/how-to-get-script-directory-in-posix-sh#29834779

But if anyone wants to take this up and give it a try, you're very welcome.