* (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.
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