Revise r272363 by collapsing the tests into a for loop.

This has the side effect of ensuring that realpath is also run for the
nominal case of PORTSDIR=/usr/ports (assuming .CURDIR is a ports directory
that relies on /usr/ports but is not rooted in it).  This ensures that any
generated PORTSDIR used is always the actual location.

Submitted by:	jkim (different implementation)