[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.
In `add_pathsep` the function `after_pathsep` also checks whether the last character is part of an multi-byte character. So not calling `add_pathsep` with `dest` (or the beginning of the last multi-byte character) could render this test useless (can there be an multi-byte character that has a path separator as his last byte?). For example, for the last multi-byte character `xx xx xx '/'` `add_pathsep` would need to append an extra '/'.