This patch updates portlint to check for the rules formulated in PR 64393,
refer to this PR for some rationale.

1.) A port is a slave port if and only if MASTERDIR != .CURDIR

2.) Slave ports should define MASTERDIR using one of the following lines:

MASTERDIR=	${.CURDIR}/../../<category>/<port>
MASTERDIR=	${.CURDIR}/../<port>

3.) Non-slave ports shouldn't define MASTERDIR at all

4.) The last line of a slave port's Makefile has to be

.include "${MASTERDIR}/Makefile"

5.) The last line of a non-slave ports Makefile must be one of:

.include <bsd.port.mk>
.include <bsd.port.post.mk>

6.) slave ports may not include bsd.port(.pre).mk

Fix: bump PORTVERSION and apply the following patch:
How-To-Repeat: 
These are detected as follows:

1.) is flagged FATAL, check with
  /usr/local/share/examples/portlint/portlintgrep 'CURDIR != MASTERDIR'

2.) and 3.) are flagged WARN, check with
  /usr/local/share/examples/portlint/portlintgrep 'define MASTERDIR'

4.) and 5.) are flagged FATAL, check with
  /usr/local/share/examples/portlint/portlintgrep 'last line of'

6.) is flagged FATAL, part of the is seen as violating 1.), check with
  /usr/local/share/examples/portlint/portlintgrep 'may not include'