(sq:vim-patch) typos and logical error.

Fix two typos.
Or (||) should be and (&&).
(sq:document) clearify
path module: export find_previous_pathsep().
Simplify ExpandRTDir().

 * Return `bool`.
 * Use concat_str and `concat_fnames` over `snprintf()`.
 * Use `fnamencmp()` over `STRNICMP()`.
 * Use `find_previous_pathsep()` from strings.h.
 * Use `vim_strcat()` over `memmove()`.
 * Remove unnecessary variable, `e`.