[Emacs-ada-mode] Main programs

[Simon Wright]

I notice that ada-mode doesn't parse 'for Main use ("main- 
program");' (actually I'm not sure whether that should be the unit  
name or the file name!). The current scheme seems to be 'assume that  
the unit in the current buffer when you load a GPR is the main  
program'; might be better to leave it up to the GPR to specify - until  
overridden by a 'Set main and build'.