telnet: Allow programatic use of telnet.

The main change is to allow input from user-specified methods,
when they are specified with CURLOPT_READFUNCTION.
All calls to fflush(stdout) in telnet.c were removed, which makes
using 'curl telnet://foo.com' painful since prompts and other data
are not always returned to the user promptly.  Use
'curl --no-buffer telnet://foo.com' instead.  In general,
the user should have their CURLOPT_WRITEFUNCTION do a fflush
for interactive use.

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

Call progress functions in telnet main loop.

Signed-off-by: Ben Greear <greearb@candelatech.com>

lib/urldata.h

@@ -1235,6 +1235,8 @@ struct UserDefined {
   curl_write_callback fwrite_header; /* function that stores headers */
   curl_write_callback fwrite_rtp;    /* function that stores interleaved RTP */
   curl_read_callback fread_func;     /* function that reads the input */
+  int is_fread_set; /* boolean, has read callback been set to non-NULL? */
+  int is_fwrite_set; /* boolean, has write callback been set to non-NULL? */
   curl_progress_callback fprogress;  /* function for progress information */
   curl_debug_callback fdebug;      /* function that write informational data */
   curl_ioctl_callback ioctl_func;  /* function for I/O control */