* nlmread.c (nlm_symtab_read): Clean up a bit.

	* nlmread.c (nlm_symtab_read):  Clean up a bit.
	* (nlm_symfile_read):  Record bounds of main() so that backtrace
	command will know where to stop.
	* objfiles.c (objfile_relocate):  Relocate entry point/func info
	for backtrace as well.
	* objfiles.h:  Define values for invalid PCs for entry point info.
	* symfile.c (init_entry_point_info):  Initialize invalid values
	with aforementioned macros.
	* config/alpha/tm-alphanw.h:  Turn on FRAME_CHAIN_VALID_ALTERNATE
	to cause backtrace to stop when it gets back to main().
	* config/i386/tm-i386nw.h:  Ditto.