warnings: fix buglets for portability warnings * lib/Automake/ChannelDefs.pm (switch_warning): Ensure the correct implications and inter-dependencies between warnings in the categories `portability', `extra-portability' and `recursive-portability' are respected. Also add detailed explicative comments, and references to the relevant tests. * tests/dollarvar2.test: Update and extend. Also, remove some unnecessary uses of `--force' option in automake calls. * tests/extra-portability3.test: New test. * tests/Makefile.am (TESTS): Add it.