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

cmdtest/environment_test.go

@@ -6,7 +6,9 @@ package cmdtest
 
 import (
 	"os"
+	"regexp"
 	"runtime"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -344,4 +346,42 @@ func TestEnvironmentVariables(t *testing.T) {
 	env = ""
 	out, err = rcloneEnv(env, "version", "-vv", "--use-json-log")
 	jsonLogOK()
+
+	// Find all the File filter lines in out and return them
+	parseFileFilters := func(out string) (extensions []string) {
+		// Match: - (^|/)[^/]*\.jpg$
+		find := regexp.MustCompile(`^- \(\^\|\/\)\[\^\/\]\*\\\.(.*?)\$$`)
+		for _, line := range strings.Split(out, "\n") {
+			if m := find.FindStringSubmatch(line); m != nil {
+				extensions = append(extensions, m[1])
+			}
+		}
+		return extensions
+	}
+
+	// Make sure that multiple valued (stringArray) environment variables are handled properly
+	env = ``
+	out, err = rcloneEnv(env, "version", "-vv", "--dump", "filters", "--exclude", "*.gif", "--exclude", "*.tif")
+	require.NoError(t, err)
+	assert.Equal(t, []string{"gif", "tif"}, parseFileFilters(out))
+
+	env = `RCLONE_EXCLUDE=*.jpg`
+	out, err = rcloneEnv(env, "version", "-vv", "--dump", "filters", "--exclude", "*.gif")
+	require.NoError(t, err)
+	assert.Equal(t, []string{"jpg", "gif"}, parseFileFilters(out))
+
+	env = `RCLONE_EXCLUDE=*.jpg,*.png`
+	out, err = rcloneEnv(env, "version", "-vv", "--dump", "filters", "--exclude", "*.gif", "--exclude", "*.tif")
+	require.NoError(t, err)
+	assert.Equal(t, []string{"jpg", "png", "gif", "tif"}, parseFileFilters(out))
+
+	env = `RCLONE_EXCLUDE="*.jpg","*.png"`
+	out, err = rcloneEnv(env, "version", "-vv", "--dump", "filters")
+	require.NoError(t, err)
+	assert.Equal(t, []string{"jpg", "png"}, parseFileFilters(out))
+
+	env = `RCLONE_EXCLUDE="*.,,,","*.png"`
+	out, err = rcloneEnv(env, "version", "-vv", "--dump", "filters")
+	require.NoError(t, err)
+	assert.Equal(t, []string{",,,", "png"}, parseFileFilters(out))
 }