sync: --fix-case flag to rename case insensitive dest - fixes #4854

Before this change, a sync to a case insensitive dest (such as macOS / Windows)
would not result in a matching filename if the source and dest had casing
differences but were otherwise equal. For example, syncing `hello.txt` to
`HELLO.txt` would result in the dest filename remaining `HELLO.txt`.
Furthermore, `--local-case-sensitive` did not solve this, as it actually caused
`HELLO.txt` to get deleted!

After this change, `HELLO.txt` is renamed to `hello.txt` to match the source,
only if the `--fix-case` flag is specified. (The old behavior remains the
default.)

fs/config.go

@@ -81,6 +81,7 @@ type ConfigInfo struct {
 	IgnoreSize                 bool
 	IgnoreChecksum             bool
 	IgnoreCaseSync             bool
+	FixCase                    bool
 	NoTraverse                 bool
 	CheckFirst                 bool
 	NoCheckDest                bool