drive: allow setting metadata to fail if failok flag is set

For example using

    --drive-metadata-permissions read,write,failok

Will allow metadata to be read and written but if the writing fails,
then only an ERROR will be written in the log and the transfer won't
fail.

backend/drive/drive.go

@@ -151,6 +151,7 @@ func (rwChoices) Choices() []fs.BitsChoicesInfo {
 		{Bit: uint64(rwOff), Name: "off"},
 		{Bit: uint64(rwRead), Name: "read"},
 		{Bit: uint64(rwWrite), Name: "write"},
+		{Bit: uint64(rwFailOK), Name: "failok"},
 	}
 }
 
@@ -160,6 +161,7 @@ type rwChoice = fs.Bits[rwChoices]
 const (
 	rwRead rwChoice = 1 << iota
 	rwWrite
+	rwFailOK
 	rwOff rwChoice = 0
 )
 
@@ -173,6 +175,9 @@ var rwExamples = fs.OptionExamples{{
 }, {
 	Value: rwWrite.String(),
 	Help:  "Write the value only",
+}, {
+	Value: rwFailOK.String(),
+	Help:  "If writing fails log errors only, don't fail the transfer",
 }, {
 	Value: (rwRead | rwWrite).String(),
 	Help:  "Read and Write the value.",