ops: refactor get_spec_reg()

Return value is bool.
errmsg (param) is bool in here and in getaltfname().
allocated (param) is bool.

src/nvim/buffer.c

@@ -2659,9 +2659,8 @@ buf_T *setaltfname(char_u *ffname, char_u *sfname, linenr_T lnum)
  * Get alternate file name for current window.
  * Return NULL if there isn't any, and give error message if requested.
  */
-char_u *
-getaltfname (
-    int errmsg                     /* give error message */
+char_u * getaltfname(
+    bool errmsg                   // give error message
 )
 {
   char_u      *fname;