[WIP/RFC] Remove third argument of concat_fnames().

Running `$ ag concat_fnames` reveals that the third argument is `true`,
except for one instance where vim_strcat() can be used instead.
For such a simple one-argument function with a perfectly good description, describing `p` feels overly verbose. Moving the description to `@param p` feels wrong, as does moving it to a `@post` block, and duplicating it in either spot again feels too verbose. Suggestions?