Allow building bin/cat on non-FreeBSD systems

[arichardson]

Summary

cat -l is needed during the installworld phase and other system's cat
don't support that flag. To avoid portability issues when compiling on
Linux/macOS (such as the the direct access to &fp->_mbstate), we
disable the entire multibyte support when building as a boostrap tool.

[emaste]

Came from rS305841.

bin/cat/cat.c

267–270

How do other systems behave here? I worry that just addressing the compilation error (_mbstate) is insufficient, and we should have a different block (or perhaps just not support multibyte encoding on !FreeBSD)?

[arichardson]

This patch was enough to make cat work enough to run installworld on Linux. But probably removing the whole wide stuff when not building for FreeBSD is better.

[brooks]

The other option here would be to write a small, portable build tool that take a file argument, opens and locks the file, and writes anything on stdin to the file until EOF.

[arichardson]

Disable all flags except -l/-u when bootstrapping

[arichardson]

Ping?

[emaste]

This still feels a little icky to me but OK.

[brooks]

This seems fine.

We got cat -l from NetBSD when I implemented the METALOG work. I do wonder if a dedicated (and easily bootstrapable) tool would make more sense.

[arichardson]

I agree that a small separate tool would be nicer. I don't really have time to write one right now though. I'll wait before committing this until it's the last blocker for cross-building. Maybe by then I'll find some time to add a write_locked (or whatever name we want to choose) bootstrap tool.

[emaste]

I'll wait before committing this until it's the last blocker for cross-building.

In fact I think it's fine to commit this now, perhaps expanding the comment before main() mentioning the alternate path. We can always revert if the other approach makes it in.