how to determine whether a port is a slave port?

[Mark Linimon]

One of the things that I would like to have available for the
database in my ports monitoring code is an indication of whether
or not a  port is a slave port.  The Porter's Handbook recommends
that the MASTERDIR makevar be used to establish that, but does
not make it clear whether its use is mandatory or not.  However,
there are over 300 ports that do not seem to use this convention.
Most seem to use the "${.CURDIR}/.." convention, although a few
seem to use PORTSDIR directly.

So, without wanting to start a bikeshed, are these just remnants
of a time before MASTERDIR was introduced?  Is there a consensus
on how slave ports ought to be handled?

mcl