Log Message:
Revert the shell used to run netpbm commands implemented as shell
scripts from bash to sh, to limit exposure to any still-unfixed or
still-undiscovered bash vulnerabilities.

A quick review of the shell scripts did not turn up any obvious
bashishms.  Hopefully they have all been fixed already, but if not,
I'd still rather have one or two scripts be broken (and volunteer to
fix any breakage reported) than have all of them be vulnerable to bash
bugs.

Approved by agc.