CamelIMAPXServer: moved handler code for untagged responses to individual functions

CamelIMAPXServer: moved handler code for untagged responses to individual functions

* this is the first step towards an extensible CamelIMAPXServer
  w.r.t. untagged IMAP responses
* there is now one handler function per untagged response
* instead of one big switch statement, there will be a
  table with pointers to the new handler function, which
  will be indexed by the untagged response code
* this will eventually eliminate the switch statement
  in imapx_untagged()
* the gperf dependency will be dropped in the process