comment added
Invalid UTF-8 chars in filename message improved
Some more work for invalid character in filename exception handling

1) Check for TAB,CR,LF in filename before everything else, as some perl modules throw additional errors/warnings in case LF in filename
2) Only TAB,CR,LF produce sane error message, other glitches (via _can_read_filename_for_mode) produces "confess" stacktrace
3) slow sanity_relative_filename performed only after _can_read_filename_for_mode
4) test fixes, it should use correct filenames inside transfer root
5) tests added for TAB,CR,LF in filenames