kern/57889: ext2fs: Drop EXT2FS_SYSTEM_FLAGS and make its behaviour the default

[rbranco%suse.de@localhost]

>Number:         57889
>Category:       kern
>Synopsis:       ext2fs: Drop EXT2FS_SYSTEM_FLAGS and make its behaviour the default
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jan 29 15:50:00 +0000 2024
>Originator:     Ricardo Branco
>Release:        NetBSD 10.0_RC3
>Organization:
>Environment:
>Description:
On Linux, only root can set the immutable & append file flags. NetBSD _should_ stick with those semantics.

The default behaviour allows the user to set the immutable/append flags, which is problematic for at least 2 reasons:

1. The user setting the flag when only root should be allowed.
1. When the user sets those flags, he cannot unset them later when running on Linux, FreeBSD (and perhaps other systems).  Only root.

FreeBSD solved this issue already in 2009:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=122047


>How-To-Repeat:

>Fix:
Drop EXT2FS_SYSTEM_FLAGS altogether and make its behaviour the