install: add FreeBSD's -U (unprivileged) option

[alexandrefresnais]

Fixes #1273

As suggested per some users, It could be nice to support FreeBSD's -U option for unprivileged install.

The manual states

-U: Indicate that install is running  unprivileged, and  that  it 
should  not  try to  change  the owner, the group, or the file
 flags of	the destination.

[alexandrefresnais]

Fixed clippy

[sylvestre]

We have an extension.md file. Could you please update it too? Thanks

[alexandrefresnais]

Sure, thank you, updated it

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[alexandrefresnais]

Skipped my test on selinux

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)