test/path: add tests for concat_fnames()
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.
vim.h: define STPCY as xstpcpy.
aktau: Return pointer to end from add_pathsep().

Allows slightly more optimal code for callers.
optimize concat_fnames()