fs: fix setting stringArray config values from environment variables

After the config re-organisation, the setting of stringArray config
values (eg `--exclude` set with `RCLONE_EXCLUDE`) was broken and gave
a message like this for `RCLONE_EXCLUDE=*.jpg`:

    Failed to load "filter" default values: failed to initialise "filter" options:
    couldn't parse config item "exclude" = "*.jpg" as []string: parsing "*.jpg" as []string failed:
    invalid character '/' looking for beginning of value

This was caused by the parser trying to parse the input string as a
JSON value.

When the config was re-organised it was thought that the internal
representation of stringArray values was not important as it was never
visible externally, however this turned out not to be true.

A defined representation was chosen - a comma separated string and
this was documented and tests were introduced in this patch.

This potentially introduces a very small backwards incompatibility. In
rclone v1.67.0

    RCLONE_EXCLUDE=a,b

Would be interpreted as

    --exclude "a,b"

Whereas this new code will interpret it as

    --exclude "a" --exclude "b"

The benefit of being able to set multiple values with an environment
variable was deemed to outweigh the very small backwards compatibility
risk.

If a value with a `,` is needed, then use CSV escaping, eg

    RCLONE_EXCLUDE="a,b"

(Note this needs to have the quotes in so at the unix shell that would be

    RCLONE_EXCLUDE='"a,b"'

Fixes #8063

fs/config/configstruct/configstruct.go

@@ -2,7 +2,7 @@
 package configstruct
 
 import (
-	"encoding/json"
+	"encoding/csv"
 	"errors"
 	"fmt"
 	"reflect"
@@ -31,7 +31,7 @@ func camelToSnake(in string) string {
 //
 // Builtin types are expected to be encoding as their natural
 // stringificatons as produced by fmt.Sprint except for []string which
-// is expected to be encoded as JSON with empty array encoded as "".
+// is expected to be encoded a a CSV with empty array encoded as "".
 //
 // Any other types are expected to be encoded by their String()
 // methods and decoded by their `Set(s string) error` methods.
@@ -58,14 +58,18 @@ func StringToInterface(def interface{}, in string) (newValue interface{}, err er
 	case time.Duration:
 		newValue, err = time.ParseDuration(in)
 	case []string:
-		// JSON decode arrays of strings
-		if in != "" {
-			var out []string
-			err = json.Unmarshal([]byte(in), &out)
-			newValue = out
-		} else {
-			// Empty string we will treat as empty array
+		// CSV decode arrays of strings - ideally we would use
+		// fs.CommaSepList here but we can't as it would cause
+		// a circular import.
+		if len(in) == 0 {
 			newValue = []string{}
+		} else {
+			r := csv.NewReader(strings.NewReader(in))
+			newValue, err = r.Read()
+			switch _err := err.(type) {
+			case *csv.ParseError:
+				err = _err.Err // remove line numbers from the error message
+			}
 		}
 	default:
 		// Try using a Set method