sieve.el: Properly close the connection on pressing 'q'

Make sieve-manage-quit close the connection and process buffer.  Also,
remove duplicate keybinding for 'q'.
sieve-manage.el: Wait for capabilities after STARTTLS

Following RFC5804, the server sends new capabilities after successfully
establishing a TLS connection with the client.  The client should update
the cached list of capabilities, but we just ignore the answer for now.
sieve.el: Put point at beginning of buffer when viewing a script