kern/59498: Add missing POSIX O_CLOFORK flag

[rbranco%suse.de@localhost]

>Number:         59498
>Category:       kern
>Synopsis:       Add missing POSIX O_CLOFORK flag
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jul 01 16:05:00 +0000 2025
>Originator:     Ricardo Branco
>Release:        NetBSD 10.99.14
>Organization:
>Environment:
>Description:
Add missing POSIX O_CLOFORK flag

It's 99% done. The TODO bits are documented. Mostly the manpages.
>How-To-Repeat:

>Fix:
https://github.com/NetBSD/src/pull/53