fs: Add string alternatives for setting options over the rc

Before this change options were read and set in native format. This
means for example nanoseconds for durations or an integer for
enumerated types, which isn't very convenient for humans.

This change enables these types to be set with a string with the
syntax as used in the command line instead, so `"10s"` rather than
`10000000000` or `"DEBUG"` rather than `8` for log level.

vfs/vfscommon/cachemode_test.go

@@ -1,6 +1,8 @@
 package vfscommon
 
 import (
+	"encoding/json"
+	"strconv"
 	"testing"
 
 	"github.com/spf13/pflag"
@@ -10,6 +12,9 @@ import (
 // Check CacheMode it satisfies the pflag interface
 var _ pflag.Value = (*CacheMode)(nil)
 
+// Check CacheMode it satisfies the json.Unmarshaller interface
+var _ json.Unmarshaler = (*CacheMode)(nil)
+
 func TestCacheModeString(t *testing.T) {
 	assert.Equal(t, "off", CacheModeOff.String())
 	assert.Equal(t, "full", CacheModeFull.String())
@@ -34,3 +39,27 @@ func TestCacheModeType(t *testing.T) {
 	var m CacheMode
 	assert.Equal(t, "CacheMode", m.Type())
 }
+
+func TestCacheModeUnmarshalJSON(t *testing.T) {
+	var m CacheMode
+
+	err := json.Unmarshal([]byte(`"full"`), &m)
+	assert.NoError(t, err)
+	assert.Equal(t, CacheModeFull, m)
+
+	err = json.Unmarshal([]byte(`"potato"`), &m)
+	assert.Error(t, err, "Unknown cache mode level")
+
+	err = json.Unmarshal([]byte(`""`), &m)
+	assert.Error(t, err, "Unknown cache mode level")
+
+	err = json.Unmarshal([]byte(strconv.Itoa(int(CacheModeFull))), &m)
+	assert.NoError(t, err)
+	assert.Equal(t, CacheModeFull, m)
+
+	err = json.Unmarshal([]byte("-1"), &m)
+	assert.Error(t, err, "Unknown cache mode level")
+
+	err = json.Unmarshal([]byte("99"), &m)
+	assert.Error(t, err, "Unknown cache mode level")
+}