Standardise on a shell dialect for scripts and reduce duplication

We are not coherent about our use of shell features, and environment.

Some of our scripts are #!/bin/bash and some are #!/bin/sh. I suggest we standardise on bash. (This presumes we do not ship any shell scripts. Any scripts we ship ought to run at least in bash and dash.)

Some of our scripts run with set -u and some without. I suggest we standardise on set -u. This will involve some code changes.

Some of our scripts are lacking set -o pipefail. We should be using this everyhwere. The need for this is one good reason to write in bash rather than sh.

When we have done this we should centralise the clone-and-hacked code (eg in tests/chutney)