syntax: c: recognize some C++ header files by their Emacs modeline

For extensionless files, recognize C/C++ ones from the mode name
between the -*- marks on the first line.

This addresses https://savannah.gnu.org/patch/?9719.

Inspired-by: Devin Hussey <husseydevin@gmail.com>

syntax/c.nanorc

@@ -1,6 +1,7 @@
 ## Here is an example for C/C++.
 
 syntax c "\.([ch](pp|xx)?|C|cc|c\+\+|cu|H|hh|ii?)$"
+header "-\*-.*\<C(\+\+)?((;|\s).*)?-\*-"
 magic "^(C|C\+\+) (source|program)"
 comment "//"