Define an implicit attribute "netbsd" internally to collect files that don't
belong to any specific attribute.

Eventually, all operations doing "foreach (files)" can be rewritten as "foreach
(attributes) foreach (files)".
Always select the fallback allattr.
Define a command to generate relocatable objects using ld -r.
Implement code to generate Makefile to build netbsd via intermediate relocatable
object files.  Disabled for now.  Commit this for further experiments.

Kernel (netbsd) has been built as:

	netbsd: *.o
		ld -o netbsd *.o

Change this to:

	netbsd: *.ko
		ld -o netbsd *.ko
	acpica.ko: ${OBJS.acpica}
		ld -r acpica.ko ${OBJS.acpica}
	:

You can call *.ko as a module, but this is not only beneficial for loadable
module, but also localize related text/data.  Various options/flags/params
will be able to be per-ko.  Unnecessary symbols can be hidden.  Many ideas
will follow.
Correct inverted logic.
Print one file per line.  Simplify code.