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.