libedit: Reduce surprising behaviour with filename completion some more:

libedit: Reduce surprising behaviour with filename completion some more:
* Quote '*', '?' and '['. While it may be more useful to expand them to
  matching pathnames, this at least matches with the completion we do.
* '@' is a regular character for filenames. Some other shells do
  @<hostname> completion but we do not.
* Prefix names starting with '-' and '+' with './' so they are not seen as
  options.