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/if_cscope.c

@@ -1841,9 +1841,7 @@ static void sig_handler(int s) {
  */
 static void cs_release_csp(size_t i, int freefnpp)
 {
-  /*
-   * Trying to exit normally (not sure whether it is fit to UNIX cscope
-   */
+  // Trying to exit normally (not sure whether it is fit to Unix cscope)
   if (csinfo[i].to_fp != NULL) {
     (void)fputs("q\n", csinfo[i].to_fp);
     (void)fflush(csinfo[i].to_fp);