imap: Updated ftpserver.pl to be more compliant, added new commands

Enriched IMAP capabilities of ftpserver.pl in order to be able to
add tests for the new IMAP features.

* Added support for APPEND - Saves uploaded data to log/upload.$testno
* Added support for LIST - Returns the contents of <reply/> section in
  the current test, like e.g FETCH.
* Added support for STORE - Returns hardcoded updated flags
* Changed handling of SELECT - Returns much more information in the
  usual set of untagged responses; uses hardcoded data from an example
  in the IMAP RFC
* Changed handling of FETCH - Fixed response format