c-c++ layer: rename default mode for header variable

New name is `c-c++-default-mode-for-headers`, possible values are
`c-mode` or `c++-mode`.

contrib/!lang/c-c++/README.md

@@ -49,11 +49,12 @@ To use this contribution add it to your `~/.spacemacs`
 ### Default mode for header files
 
 By default header files are open in `c-mode`, you can open them in `c++-mode`
-by setting the variable `c-c++-use-c++-mode-for-dot-h` to `t`.
+by setting the variable `c-c++-default-mode-for-headers` to `c++-mode`.
 
 ```elisp
 (setq-default dotspacemacs-configuration-layers
-  '((c-c++ variables: c-c++-use-c++-mode-for-dot-h t)))
+  '((c-c++ variables:
+           c-c++-default-mode-for-headers 'c++-mode)))
 ```
 
 **Note** To set the variable for a given project, create a directory local

contrib/!lang/c-c++/config.el

@@ -18,5 +18,5 @@
 (spacemacs|defvar-company-backends c-mode-common)
 (spacemacs|defvar-company-backends cmake-mode)
 
-(defvar c-c++-use-c++-mode-for-dot-h nil
-  "If non nil then use c++-mode when opening .h files.")
+(defvar c-c++-default-mode-for-headers 'c-mode
+  "Default mode to open header files. Can be `c-mode' or `c++-mode'.")

contrib/!lang/c-c++/packages.el

@@ -33,8 +33,7 @@
   (use-package cc-mode
     :defer t
     :init
-    (when c-c++-use-c++-mode-for-dot-h
-      (add-to-list 'auto-mode-alist '("\\.h$" . c++-mode)))
+    (add-to-list 'auto-mode-alist `("\\.h$" . ,c-c++-default-mode-for-headers))
     :config
     (progn
       (require 'compile)