make language_of_main static

This makes the global language_of_main static.  Now it can be set only
via a new argument to set_main_name.

2014-01-15  Tom Tromey  <tromey@redhat.com>

	* dbxread.c (process_one_symbol): Update.
	* dwarf2read.c (read_partial_die): Update.
	* symfile.c (set_initial_language): Call main_language.
	* symtab.c (language_of_main): Now static.
	(set_main_name): Add 'lang' parameter.
	(find_main_name): Update.
	(main_language): New function.
	(symtab_observer_executable_changed): Update.
	* symtab.h (set_main_name): Update.
	(language_of_main): Remove.
	(main_language): Declare.