   This server use the CON_FILT.PSW file, that have this format:
   Every empty line, or line that start with the '#' are ignored.

   The first valid line are password request prompt.

   The second line is the filter mode, 0 for OPEN SYTEM, 1 for CLOSED SYSTEM,
   2 for RO SYSTEM. A second number may follow, 0 for MD2 sequence only,
   1 for 5 numbers followed by MD2 sequence.

   The others line are the callsign, without ssid, flag and its password, like:
   CALL FLAG PASSWORD
   CALL FLAG PASSWORD
   ... etc.

   NOTE: PASSWORD is not needed if FLAG=0


   OPEN SYSTEM MODE
   If the call is not declared in the con_filt.psw file, the user will be
   connected without password request, userful for bbs forward, or user that
   do not want password.
   If a callsign is declared:
   if FLAG=0, the callsign is excluded, disconneted.
   if FLAG=1, the callsign is connected after the password request.

   CLOSE SYSTEM MODE
   Only the callsign declared in the con_filt.psw may access to the system,
   the other will be disconneted.
   For declared callsign:
   if FLAG=0, the connection is accepted without password request.
   if FLAG=1, the connection is accepted, but the password will be requested.

   RO SYSTEM MODE
   The callsign declared in the con_filt.psw may access to the system,
   the other will only have a read-only access.
   For declared callsign:
   if FLAG=0, the connection is accepted without password request.
   if FLAG=1, the connection is accepted, but the password will be requested.

   >  Prompt must be long at most 80 characters
   >  Password my be long at most 255 characters, without space.
   >  This program, if renamed C_FILTER.EXE, run also under FBB 5.15a and upper

   The server use a temporary file XXXXXX.CFT, where XXXXXX is the callsign of
   the user, to store the calculated password and date time of any unsuccesful
   logon; you do not need to touch this file, all management will be done by
   con_filt; the temporary file will be automatically deleted when not more
   needed.

   The server use both the standard 5 letters password and the MD2 (c)RSA
   alghoritm. The user may answer the exact password, in standard or md2 mode, 
   the server will decode automatically the corrected mode.

