lib/fs: Properly handle case insensitive systems (fixes #1787, fixes #2739, fixes #5708)

With this change we emulate a case sensitive filesystem on top of
insensitive filesystems. This means we correctly pick up case-only renames
and throw a case conflict error when there would be multiple files differing
only in case.

This safety check has a small performance hit (about 20% more filesystem
operations when scanning for changes). The new advanced folder option
`caseSensitiveFS` can be used to disable the safety checks, retaining the
previous behavior on systems known to be fully case sensitive.

Co-authored-by: Jakob Borg <jakob@kastelo.net>

lib/ur/usage_report.go

@@ -269,6 +269,9 @@ func (s *Service) reportData(ctx context.Context, urVersion int, preview bool) (
 			}
 			report.FolderUsesV3.BlockPullOrder[cfg.BlockPullOrder.String()]++
 			report.FolderUsesV3.CopyRangeMethod[cfg.CopyRangeMethod.String()]++
+			if cfg.CaseSensitiveFS {
+				report.FolderUsesV3.CaseSensitiveFS++
+			}
 		}
 		sort.Ints(report.FolderUsesV3.FsWatcherDelays)