config: make config file system pluggable

If you are using rclone a library you can decide to use the rclone
config file system or not by calling

    configfile.LoadConfig(ctx)

If you don't you will need to set `config.Data` to an implementation
of `config.Storage`.

Other changes
- change interface of config.FileGet to remove unused default
- remove MustValue from config.Storage interface
- change GetValue to return string or bool like elsewhere in rclone
- implement a default config file system which panics with helpful error
- implement getWithDefault to replace the removed MustValue
- don't embed goconfig.ConfigFile so we can change the methods

fs/config/default_storage.go

@@ -0,0 +1,61 @@
+package config
+
+// Default config.Storage which panics with a useful error when used
+type defaultStorage struct{}
+
+var noConfigStorage = "internal error: no config file system found. Did you call configfile.LoadConfig(ctx)?"
+
+// GetSectionList returns a slice of strings with names for all the
+// sections
+func (defaultStorage) GetSectionList() []string {
+	panic(noConfigStorage)
+}
+
+// HasSection returns true if section exists in the config file
+func (defaultStorage) HasSection(section string) bool {
+	panic(noConfigStorage)
+}
+
+// DeleteSection removes the named section and all config from the
+// config file
+func (defaultStorage) DeleteSection(section string) {
+	panic(noConfigStorage)
+}
+
+// GetKeyList returns the keys in this section
+func (defaultStorage) GetKeyList(section string) []string {
+	panic(noConfigStorage)
+}
+
+// GetValue returns the key in section with a found flag
+func (defaultStorage) GetValue(section string, key string) (value string, found bool) {
+	panic(noConfigStorage)
+}
+
+// SetValue sets the value under key in section
+func (defaultStorage) SetValue(section string, key string, value string) {
+	panic(noConfigStorage)
+}
+
+// DeleteKey removes the key under section
+func (defaultStorage) DeleteKey(section string, key string) bool {
+	panic(noConfigStorage)
+}
+
+// Load the config from permanent storage
+func (defaultStorage) Load() error {
+	panic(noConfigStorage)
+}
+
+// Save the config to permanent storage
+func (defaultStorage) Save() error {
+	panic(noConfigStorage)
+}
+
+// Serialize the config into a string
+func (defaultStorage) Serialize() (string, error) {
+	panic(noConfigStorage)
+}
+
+// Check the interface is satisfied
+var _ Storage = defaultStorage{}