Revise file rotation to prevent overwriting existing files

[cliffmccarthy]

• This fixes File rotation can overwrite existing files #56.
• Moved logic for computing rotated filenames into a dedicated set of functions that handle this for various command-line scenarios. If we were designing this functionality from scratch, we would do this differently, with more simplicity and consistency. This implementation maintains general compatibility with the behaviour as of version 0.9.73.
• No generated filenames will be used that overwrite an existing file. This implementation checks any generated filename before using it, and as per the documentation, the only filename that should ever be overwritten is the exact name provided on the command line, if the 'filename' argument was used.
• The --leading-zeros value is capped at 20, the number of decimal digits required to format a 64-bit integer.
• This partially addresses the double-extension problem described in Fileprefix is not computed correctly on file-rotation #20. It does not address the issue in cases where 'filename' was specified on the command line, because in that case it treats the filename as an opaque string and does not attempt to insert the rotation number inside that string.