security/ca_root_nss: Restore the ETC_SYMLINK.

It turns out that some ports have an undisclosed dependency on the
symlink and cannot be trivially changed to use the system trust
store instead.

Amend the package message to make it clear that software which relies
on this symlink is not following recommended practice.

I will look into getting certctl(8) to provide cert.pem instead, but
it may take a while until we can rely on this being in place on all
supported releases.

This partly reverts commit 483e74f44b82.

PR: 274322
MFH: 2023Q4
Reviewed by: fluffy
Differential Revision: https://reviews.freebsd.org/D42120