ports/158179: some packages do not fully honor -P dir option in pkg_add(1)

[Dieter BSD]

> 1.  Spell out very clearly its purpose - is it to populate a jail, for
> example?

Populating a chroot/jail is one purpose.

Another is to test a new version of a package without messing up the
existing version.

There are bound to be other reasons.

Compiling the port rather than installing a package isn't always
a solution.  Many ports are incredibly difficult to get compiled.

PREFIX needs to be an install time variable, not compile time.