Automatically detect whether .h file is C or C++

Automatically detect whether .h file is C or C++

* lisp/progmodes/cc-mode.el (c-or-c++-mode): A new function which
analyses contents of the buffer to determine whether it looks like C++
source code and based on that enables c-mode or c++-mode.
(c-or-c++-mode--regexp): Regular expression which, when matches
a buffer, signals file is C++.