Compare logfiles for tests.

Compare logfiles for tests.

- Add -dseparate-log-files. This writes BASENAME.log for every file
processed. The "failed files" notice is written to the default log.

- Process .log files in output-distance too

- Use difflib to measure differences in textfiles

- Add ok-test target to toplevel make file. This moves the out-test
directory to out-test-ok, which is now the base for comparisons.

- Document this procedure in INSTALL.texi