Bug 11609 – Use better substituted characters in coverage output filenames

Currently, if dmd -cov is invoked on an absolute pathname, e.g., /mnt/1/src/mysource.d, then when you run the program, it produces the coverage report in a file named -mnt-1-src-mysource.d. The use of '-' here as a substitute character for '/' is unfortunate, because it interacts badly with Unix tools. E.g., you have to type: rm -- -mnt-1-src-mysource.d instead of just: rm -mnt-1-src-mysource.d because the initial '-' gets misinterpreted as an option. Suggestion: use '_' instead of '-' as the substitute character.

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