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/contract/contract.go

@@ -128,6 +128,7 @@ type Report struct {
 		DisableFsync            int            `json:"disableFsync,omitempty" since:"3"`
 		BlockPullOrder          map[string]int `json:"blockPullOrder,omitempty" since:"3"`
 		CopyRangeMethod         map[string]int `json:"copyRangeMethod,omitempty" since:"3"`
+		CaseSensitiveFS         int            `json:"caseSensitiveFS,omitempty" since:"3"`
 	} `json:"folderUsesV3,omitempty" since:"3"`
 
 	GUIStats struct {