misc: UNIX => Unix #4022

Although UNIX is a registered trademark of The Open Group, it doesn't
really matter whether we refer to these systems as UNIX, Unix, or
Unix-like. So, for consistency, refer to them collectively as Unix.

Related:
http://www.greens.org/about/unix.html
http://www.unixica.com/html/unixunix.html

src/nvim/path.c

@@ -196,7 +196,7 @@ char_u *get_past_head(char_u *path)
 int vim_ispathsep(int c)
 {
 #ifdef UNIX
-  return c == '/';          /* UNIX has ':' inside file names */
+  return c == '/';          // Unix has ':' inside file names
 #else
 # ifdef BACKSLASH_IN_FILENAME
   return c == ':' || c == '/' || c == '\\';