Log Message:
pkgtools/pkglint: update to 20.1.12

Changes since 20.1.11:

The file bsd.pkg.mk must only ever be included by package Makefiles
directly, not by other Makefile fragments.  Seen in www/w3m.

The variable BUILDLINK_PREFIX.* should only be used for packages that
have actually been included by the package.  This catches the use of
BUILDLINK_PREFIX.libiconv, which should have been iconv instead.

Allow comments before line 3 in buildlink3.mk files. This is necessary
for mariadb55-client since its buildlink identifier is mysql-client,
which is so non-obvious that it needs to be documented.