Provide intuitive tab-completion when using prompt and rejecting preferred handler

[thomp]

Improve handling of situation where (1) a preferred handler is defined internally for x.txt (2) x.txt is selected in dired and launched with prompt ('K') (3) user prefers not to use preferred handler

Expected: tab-completion should provide other options -- other preferred handlers or default list of available programs -- or attempt to complete if partial name of a handler is provided (e.g., "libre" should, if no match in preferred handler, trigger completion via completions derived from larger set of potential executables)

Reality: in this situation, tab-completion is currently less-than-helpful