Remove Windows support

Unfortunately, I don't have the expertise or bandwidth to maintain
the Windows support in Litestream. I'm open to re-adding support in
the future but right now it is hindering development and is not
well-tested or well-used.

internal/internal.go

@@ -132,7 +132,7 @@ func MkdirAll(path string, mode os.FileMode, uid, gid int) error {
 
 	if j > 1 {
 		// Create parent.
-		err = MkdirAll(fixRootDirectory(path[:j-1]), mode, uid, gid)
+		err = MkdirAll(path[:j-1], mode, uid, gid)
 		if err != nil {
 			return err
 		}
@@ -154,6 +154,15 @@ func MkdirAll(path string, mode os.FileMode, uid, gid int) error {
 	return nil
 }
 
+// Fileinfo returns syscall fields from a FileInfo object.
+func Fileinfo(fi os.FileInfo) (uid, gid int) {
+	if fi == nil {
+		return -1, -1
+	}
+	stat := fi.Sys().(*syscall.Stat_t)
+	return int(stat.Uid), int(stat.Gid)
+}
+
 // ParseSnapshotPath parses the index from a snapshot filename. Used by path-based replicas.
 func ParseSnapshotPath(s string) (index int, err error) {
 	a := snapshotPathRegex.FindStringSubmatch(s)