Add bsdcat tests

First commits fix misc issues and inconsistencies I've discovered while writing tests, namely:

* Keep gitignore alpha-sorted, add `cat/.deps/`, `cat/.dirstamp`.
* `$(bsdcat_EXTRA_DIST)` line was missing backslash in Makefile.am, but doesn't cause any errors because it's empty (until last commit with tests).
* Looking at test_help.c, updated `bsdcat --help` to exit with 0 and use stdout (instead of 1/stderr).
* Initial bsdcat version printed errors about missing/bogus arguments to stderr, but didn't set resulting exit code to non-zero, addressed that in 6eeca18.

And the last commit contains the tests - for archive formats I could easily produce with other tools at the moment, interface and exit statuses.