Log Message:
pkgtools/pkglint: update pkglint to 19.3.15

Changes since 19.3.14:

Invalid lines in PLIST files are now reported as errors instead of
warnings. If pkglint doesn't know about it, it must be an error.

In PLIST files, all paths are validated to be canonical. That is, no
dotdot components, no absolute paths, no extra slashes, no intermediate
dot components.

Fewer notes for unexpanded variable expressions in DESCR files. Before,
the text $@ was reported as possible Makefile variable even though it
was just a Perl expression.

README files are allowed again in pkgsrc package directories. There was
no convincing argument why these should be forbidden.

A few diagnostics have been changed from NOTE to WARNING or from WARNING
to ERROR, to match their wording.

When pkglint suggests to replace :M with ==, the wording is now "can be
made" instead of "should".