move main name into the progspace

This moves the "main" name and language into an object attached to the
current progspace.  This prevents problems if there are multiple
inferiors tha have different ideas of "main" -- which matters at least
for unwinding, see frame.c:inside_main_func.

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

	* symtab.c (main_progspace_key): New global.
	(struct main_info): New.
	(name_of_main, language_of_main): Remove.
	(get_main_info, main_info_cleanup): New function.
	(set_main_name, main_name, main_language): Use get_main_info.
	(_initialize_symtab): Initialize main_progspace_key.