Factor new vfs module out of cmd/mountlib

This is an OS style file system abstraction with directory caching
used in mount, cmount, serve webdav and serve http.

vfs/errors.go

@@ -0,0 +1,39 @@
+// Cross platform errors
+
+package vfs
+
+import "fmt"
+
+// Error describes low level errors in a cross platform way
+type Error byte
+
+// NB if changing errors translateError in cmd/mount/fs.go, cmd/cmount/fs.go, cmd/serve/webdav/webdav.go
+
+// Low level errors
+const (
+	OK Error = iota
+	ENOENT
+	ENOTEMPTY
+	EEXIST
+	ESPIPE
+	EBADF
+	EROFS
+)
+
+var errorNames = []string{
+	OK:        "Success",
+	ENOENT:    "No such file or directory",
+	ENOTEMPTY: "Directory not empty",
+	EEXIST:    "File exists",
+	ESPIPE:    "Illegal seek",
+	EBADF:     "Bad file descriptor",
+	EROFS:     "Read only file system",
+}
+
+// Error renders the error as a string
+func (e Error) Error() string {
+	if int(e) >= len(errorNames) {
+		return fmt.Sprintf("Low level error %d", e)
+	}
+	return errorNames[e]
+}