vfprintf(3): Add support for kernel %b format

[cem]

Summary

This is a direct port of the kernel %b format.

I'm unclear on if (more) non-portable printf extensions will be a problem. I
think it's desirable to have userspace formats include all kernel formats, but
there may be competing goals I'm not aware of.

I think das@ should review this as the most heavy printf committer, but they
don't appear to have a phabricator account.

Test Plan

#include <stdio.h>
void main(void) {
        printf("%b\n", 0x12345, "\20\01low\02med\21hi");
}

Should print: 12345<low,hi>

[emaste]

I think there are lots of reasons we'd like to compile and use kernel code in userspace and having %b be available is certainly useful for that. On the other hand, if this becomes widely used in FreeBSD userland code it makes it harder to use that code in other projects or possibly build FreeBSD on other hosts.

Do you have an example use case in mind?

[cem]

In D8426#175349, @emaste wrote:

I think there are lots of reasons we'd like to compile and use kernel code in userspace and having %b be available is certainly useful for that. On the other hand, if this becomes widely used in FreeBSD userland code it makes it harder to use that code in other projects or possibly build FreeBSD on other hosts.

Yep!

Do you have an example use case in mind?

Yeah. At $WORK we've got an on-disk structure that is manipulated by userspace. It's got a few flags fields. We have a tool that prints out the on-disk structure in a human readable format. It'd be convenient to use %b, rather than re-implement %b, in that tool.

(I guess we ($WORK) could always carry it as a local libc patch — it's very self-contained. But I prefer not to do that if possible.)

[cem]

Any strong objection to this?