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.

Use .if exists(), for four specific possibilities relative to .CURDIR:
., .., ../.., and ../../..  The fourth possibility is primarily in case
ports ever grows a third level.  If none of these paths exist, fall back to
the old default of /usr/ports.

This removes the need to set PORTSDIR explicitly (or via wrapper script) if
one is running out of a ports tree that is not in /usr/ports, but in a
home directory.

Reviewed by:	bapt, bdrewery (older version)
CR:		D799
MFC after:	1 week
Sponsored by:	Spectra Logic