Change imap_cmd_start() to return OK when the cmd_queue is finished. (closes #3956)

Change imap_cmd_start() to return OK when the cmd_queue is finished. (closes #3956)

Some response handlers can end up recursively calling
imap_cmd_start(), processing all the command completions.  If the
outer call was an imap_exec(), this would result in the loop never
being terminated (or just blocking reading a server that has already
finished all the commands).

Change the callers that are simply using it to read a response,
without having called cmd_start(), to check for IMAP_CMD_OK instead.
Currently this is just the open connection function.