- removed all warnings to make the software compile with -Wall

- bug in general imap handling

martin@askja:~$ telnet mail 143
Trying 10.0.0.1...
Connected to mail
Escape character is '^]'.
* OK mail Cyrus IMAP4 v2.0.17 server ready
A001 LOGIN user pass
A001 OK User logged in
A001 STATUS INBOX (MESSAGES)
A001 NO System I/O error

problem:

   * imap_checkmbox() exits with BYE(...) -> socket is closed and freed
   * imap_goodbye() is called, closes and frees the socket a second time
      -> crash because of duplicate free


workaround:

   * set socket pointer to NULL after each Sclose()
   * imap_goodbye() does not access a socket whose pointer is NULL

   (to address similar issues)
   * imap_checkmbox() does not run if mb->status is not equal
      STAT_IDLE | STAT_RUNNING

   generally, the use of mb->status seems confusing to me

SSL support for imap

- generic approach -> no new mailbox type

- two different types of imap ssl servers
   * separate port for imapssl
   * standard imap server offers STARTTLS command to continue an
     existing session via ssl
   -> this difference should not have to be configured by the user

- two new options in asmailrc
   ssl yes -> new flag in mbox_struct->flags
   trustedCaDir /my/dir/   new component mbox_struct->trustedCaDir

   -> added an example to asmailrc.sample

- openssl availability is checked by configure
   define HAVE_OPENSSL_SSL_H in config.h is used to distinguish ssl and
   non-ssl parts

   (code could be optimized to use fewer #ifdef statements)
   
- new functions in socklib.c

   * Sslclient()
      switch an open connection created with Sclient() to SSL
   * Sslread()
      read a single line, should behave exactly like Sread()

      uses a temporary buffer to store data that was read from ssl and
      decrypted, data from this buffer is returned to the caller

      this buffer and its size are parts of the socket structure

   * Sslwrite()

- imap.c

   * many different cases for imap_login()
      - no ssl support
      - ssl supported, non-ssl session configured
      - ssl supported, ssl configured, imapssl server
      - ssl supported, ssl configured, plain imap server with STARTTLS
        support
   
      handling of ssl supported, ssl configured
         * assume server is imapssl
         * if this fails, close session and re-open it
         * assume session is plain imap
         * send a STARTTLS command
         * if we get a positive answer, switch session to ssl
         * otherwise terminate session

   * macro WAIT_OK uses either Sslread() or Sread()
      no other adjustments in imap_checkmbox(), both Sread() and
      Sslread() should behave identically

   * new macro WRITE_OUTPUT to write, using either Swrite() or Sslwrite()


- verification of the server certificate

   is done in Sslclient()
   print only a warning if verification fails

   trustedCaDir /my/dir/ option in asmailrc

   /my/dir contains a .pem file for each certificate authority we trust
   the directory has to be processed for use with openssl:
      go to the directory and run c_rehash .


   (I could provide more infos about how to create the CA's certificate
   and how to sign a client certificate with it)





BUGS:

martin@askja:~$ telnet localhost 143
Trying 127.0.0.1...
Connected to askja.kaiser.cx.
Escape character is '^]'.
* OK Dovecot ready.
a001 LOGIN
a001 BAD Error in IMAP command received by server.
(long time of inactivity)
* BYE Disconnected for inactivity.
Connection closed by foreign host.

-> BYE(STAT_CONN) crashes (broken pipe) because the server has already
   closed the connection