[WIP] 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.
Good point. Aside from that, I've been working on a change to make `add_pathsep()` take a base and end pointer (maybe changing it to `add_pathsep_at()`), though I might like to go even a step further and abstract `add_pathsep()` out of most situations. The current interface makes optimization painful and error prone.