Simplify Linux-DOSEMU door setup

Allows %f (drop file) and %s (startup-directory) to be specified on the external program's command-line and expand to the paths that DOSEMU will understand. No more hard-coding "D:\" or "E:\" and the drop filename on the command-line required.

Also (for all builds), include the door's startup directory for %s specifier on the clean-up command-line. This just seems like an oversight.