Mk/bsd.port.mk: Clarify UNAME_r failure.

An example of this happening is with recent Poudriere changes:

• Poudriere <= 3.3 used to set UNAME_r in /etc/login.conf's environment during *jail creation*.
• Poudriere >= 3.4 only sets it in the environment during *jail runtime*.

Creating a jail with Poudriere 3.4 and then using Poudriere 3.3 on it
would fail to have UNAME_r properly set. It would read OSVERSION from
/usr/include/sys/param.h to find the installed files release. The ports
framework, and builds, need uname to be correct for the *jail/chroot*
and not be of the running kernel.

Add a footshoot enabler while here which should not be used for any
reason as it can create packages targetting the release of the running
kernel rather than the chroot/jail's expected version.