break one way or another. With it goes the package-loop and the
describe loop.
(2) Add new variable MASTERDIR to make it easier to share files between ports.
bsd.port.mk will find things like ${PKGDIR} underneath
${MASTERDIR} (which defaults to ${.CURDIR}).
(3) Do not allow MD5_FILE to be renamed. Funny things can happen if
you do that.
(4) Use a few more absolute paths in the bsd.port.pre.mk part. I
can't use absolute paths for sysctl because it moved recently.
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