pkgtools/pkglint: update to 19.3.16

Changes since 19.3.15:

When a package-settable variable gets a default value using the ?=
operator, pkglint no longer suggests to include bsd.prefs.mk, since that
doesn't make sense. Including bsd.prefs.mk only defines user-settable
and system-provided variables.

User and group names may be a single character only. While not widely
used, it's syntactically valid and there's no reason to prevent this.

In variable assignments, when pkglint removes unnecessary whitespace
between the variable name and the operator, it keeps the indentation of
the variable value the same as before. Previously, the indentation had
been changed, which required another run of pkglint --autofix.

PREFIX can only be used as a replacement for LOCALBASE after the whole
package Makefile has been loaded. This is because PREFIX is defined
very late, by bsd.pkg.mk. Therefore, don't suggest to replace LOCALBASE
with PREFIX in .if conditions.

When pkglint suggests to replace INSTALL_DATA_DIR commands with setting
INSTALLATION_DIRS instead, paths with a trailing slash are correctly
looked up in the PLIST. This suggests to use AUTO_MKDIRS more often.

(rillig)