bin/59971: dd(1): POSIX.1-2024 updates

["campbell+netbsd%mumble.net@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         59971
>Category:       bin
>Synopsis:       dd(1): POSIX.1-2024 updates
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Feb 07 20:00:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, 9, ...
>Organization:
The NetDD Foundation, Inc.
>Environment:
>Description:
https://pubs.opengroup.org/onlinepubs/9799919799.2024edition/utilities/dd.html

We appear to be missing iflags=fullblock, not sure if anything else.

>How-To-Repeat:
try to figure out why src/distrib/utils/embedded/mkimage copies a single byte at a time, up to 4 MB, with dd bs=1, to pad an image with zeroes
>Fix:
Yes, please!