operate on all detected hardlinks

[nbeisert]

fixes #109

description of issue: secondary files with identical dev/inode pairs are dropped in favor of the highest ranking file within a group of identicals and thus removed from consideration early on in the selection process.

description of changes: keep a list of path references to identical dev/inodes as std::vector m_aliases in Fileinfo; apply all operations (print, delete, symlink, hardlink) to aliases as well.

considerations:

• might further unify delete, symlink and hardlink operations into one worker function (alike transactional_operation) with callback to desired operation.
• a command line option for the modified behaviour appears unnecessary because rdfind misses existing hard links (need to run it twice) #109 feels more like a bug rather than a feature request; I cannot think of a situation where the secondary references should not be handled in the same way as the highest ranking one (unless further criteria for selection are added).
• this appears to address the same issue as Added -rememberidentinode option which better handles hardlink groups #68 albeit with a different approach.