Log message:
pkgtools/check-portability: check extracted files for portability issues

Checks whether the given files use features of programming languages that
are not portable to a wide range of platforms.

See mk/check/check-portability.mk.

https://mail-index.netbsd.org/tech-pkg/2020/03/10/msg022787.html

This new check is not active by default, and the package is not forced to
be installed. This will be done after getting some experience in
practical cases.

The code has been tested by extracting about 1.7 GB of pkgsrc distfiles
and scanning for lines containing both "[[" and "]]".