Move nm.c cached line number info to bfd usrdata

Move nm.c cached line number info to bfd usrdata

Replace the static variables used by nm to cache line number info
with a struct attached to the bfd.  Cleaner, and it avoids any concern
that lineno_cache_bfd is somehow left pointing at memory for a closed
bfd and that memory is later reused for another bfd, not that I think
this is possible.  Also don't bomb via bfd_fatal on errors getting
the line number info, just omit the line numbers.

	* nm.c (struct lineno_cache): Rename from get_relocs_info.
	Add symcount.
	(lineno_cache_bfd, lineno_cache_rel_bfd): Delete.
	(get_relocs): Adjust for struct rename.  Don't call bfd_fatal
	on errors.
	(free_lineno_cache): New function.
	(print_symbol): Use lineno_cache in place of statics.  Don't
	call bfd_fatal on errors reading symbols, just omit the line
	info.
	(display_archive, display_file): Call free_lineno_cache.