Add warning before attemping hardlink operations on extfat filesystems (which does not support them)

[michaelerule]

rdfind 1.5.0 will happily run on an extfat filesystem with "-makehardlinks true" arguments. Hours later, it will then fail (because extfat file systems cannot have hard or soft links), providing the user with a list of "Failed to make hardlink" errors and no further hints. "Maybe this drive just can't do hardlinks" was my third guess, after "maybe there's something wrong with the mount or directory settings?" and "maybe there is some maximum number of hardlinks it can support?". More informative messages could save less-experienced users like myself hours of troubleshooting time.

Suggested changes are mostly related to text printed to the user and not the behavior of rdfind:

• Attempt to detect file system(s)
• Warn users before running that operation will likely fail (or if file system could not be determined), e.g. when combining "-makehardlinks true" with an extfat filesystem
• If subsequent operations fail, print informative hints to the user rather than the generic "Failed to make hardlinks" output.