ftguess: add option to rename files with extension matching format

[decalage2]

This is mostly useful when downloading malware samples that have filenames without extension, e.g. just a file hash. ftguess could guess the file type and add the corresponding file extension (only when it is missing or incorrect).
If one wants to avoid the file to be opened by mistake, it could alternatively add something after the extension (for example ".doc_" instead of ".doc").
This would be even more useful if ftguess could process several files using wildcards.