r272282:
  Search for the nearest PORTSDIR where Mk/bsd.ports.mk exists, from .CURDIR.
  This will only take effect if PORTSDIR is not set, as previously supported.

r272363:
  Always resolve PORTSDIR to absolute paths using realpath(1).

r272383:
  Revise r272363 by collapsing the tests into a for loop.