Log Message:
pkgtools/check-portability: update to 19.4.1

Changes since 19.4.0:

* Makefile.am and Makefile.in are checked even though they don't start
  with a #! line.
* Only shell programs with an interpreter that is clearly a POSIX shell
  are checked. Before, there was a blacklist of interpreters to be
  skipped.
* Lots of refactorings and code cleanups.
* Some additional test files.