In d2d_runner, we don't properly check which commands support --stringparam

d2d_runner allows adding --stringparams via a file listing them. Unfortunately, not all supported daps subcommands allow --stringparam, namely package-{pdf,html,src} and online-docs don't.

This is not currently a practical issue for docserv2 because it only uses pdf, html, single-html, and epub. It's also not an issue for anyone using daps2docker normally because the --stringparam thing is entirely unsupported there, only d2d_runner knows how to use that.