Add "enum" type for config options

[flashcode]

Feature description

Today options can be with type "integer" and have "string values": that means they are stored as integer internally but set and shown as strings.

This is confusing and not even consistent, for example /help irc.look.nick_mode shows:

Option "irc.look.nick_mode":
  description: display nick mode (op, voice, ...) before nick (none = never, prefix = in prefix only (default), action = in action messages only, both = prefix + action messages)
  type: string
  values: "none", "prefix", "action", "both"
  default value: "prefix"
  current value: "prefix"

While fset shows the option as integer:

irc.look.nick_mode         integer  prefix

Behavior of type "enum":

• Works like the "integer": stored as integer, set and shown with a fixed list of strings.
• For compatiblity, any new option with type "integer" and string values will be automatically created as type "enum".
• If a new option is created with type "enum" and no string values, this is considered an error and the option is not created.
• /help and fset plugin show type "enum".

Once implemented, the display of /help irc.look.nick_mode will be:

Option "irc.look.nick_mode":
  description: display nick mode (op, voice, ...) before nick (none = never, prefix = in prefix only (default), action = in action messages only, both = prefix + action messages)
  type: enum
  values: "none", "prefix", "action", "both"
  default value: "prefix"
  current value: "prefix"

Fset plugin will show:

irc.look.nick_mode         enum  prefix