telnet: Allow programatic use of telnet via libcurl.
   The main change is to allow input from user-specified methods,
   but in order to be backwards compatible, the user also has to set
   the option CURLOPT_FORCE_USE_READFUNC to true.

   Also fix assumption that reading from stdin never returns < 0.
   Old code could crash in that case.

   Call progress functions in telnet main loop.

Patch is attached, please let me know if it needs any improvements.

-- 
Ben Greear <greearb_at_candelatech.com>
Candela Technologies Inc  http://www.candelatech.com

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